Continuous Flow Fuel Injection System Maintenance
Repair Stations Threatened
Part 145 recommendations may shut you
By Stephen P. Prentice
March 2000
If you work for or contract with a repair station, you should note that the Feds have proposed changes to the venerable FAR 145 Repair Station rules. They state that they have proposed these changes as being necessary because ". . . many portions of the current repair station regulations do not reflect changes in repair station business practices and aircraft maintenance practices, or advances in aircraft technology."
I don't know about you, but to me, business practices seem to have remained about the same for many years. The only exceptions I see are upgrades in equipment and a few computers added here and there.
Nonetheless there seems to be this big push to put more pressure on small repair stations to changes their work practices — changes that will do nothing but increase their costs of doing business.
Key Changes
The key changes involve new requirements relevant to station Ratings and Classes, relative to work performed.
Added responsibility for subcontracted work is also a big part of the new proposed rule. It would place direct responsibility more clearly on the repair station for the work of the contractor. They will be required to audit, assure quality control, and verify any work performed. Repair stations will be held strictly responsible for all aspects of the work. Required manuals would be expanded to include further inspection, training and procedural outlines for all work.
Training would be enhanced and include an FAA approved program for initial and recurrent training for all employees using an FAA approved training manual.
As a technician or other employee at a repair station, you should know that many in our industry believe that these changes will have a serious, negative impact on existing repair stations.
Many feel they will be forced to relinquish their status as repair stations, or worse, just go out of business and shut the doors. It's clear that the administrative and training requirements alone may be sufficient to sink many of them. Add to this, other reporting requirements and the already existing drug testing protocols and liability insurance requirements, and you have a guaranteed formula for disaster.
Certified Repair Stations
The theory behind a certified shop is that it is somehow on a higher level and does better work than a non-certified shop. In reality, under the current law there should be no difference in the quality of the work performed.
The subtle difference, of course, is all too obvious. In one case, the repair station approves aircraft for return to service and in the other, an individual mechanic does the approving. This insulates, to a certain extent, the mechanics working at a repair station and puts the other more directly under the general liability and FAA gun.
Some have wondered and still wonder why we need repair stations in the first place? Well, there are some advantages. If you keep in mind that a repair station can have many unlicensed personnel on the payroll, you can begin to see some of the reasoning.
A repair station can have one or two licensed personnel and many more unlicensed workers — the number can vary up and down over a period of time. Unlicensed people, with some exceptions, have to be supervised. How many workers can an A&P supervise?
Of course, FAA is supposed to have continuous oversight of certified facilities but as we all know from recent history, it just does not happen. So, repair stations may have many unlicensed workers and thus cut down on their payroll requirements.
Additionally, a repair station can have certificated "repairmen." As long as the non-licensed person and/or the repairman work under the "supervision" of licensed personnel, they are legal. Problem is, how much supervision do these people need?
Further, an advantage of a repair station is that it does not have to deal with the 337 forms. As stated quite clearly in the FAR (App. B, Part 43), a certificated repair station need only use the customer's work order to document the repair or alteration. That's it.
Finally, one of the hallmarks of a certified shop is the continuous surveillance of the operation by the FAA. Today, each shop has an assigned principal maintenance inspector who regularly audits the facility for quality control.
The Non-Certified Shop
There are substantially more maintenance shops in this country that are "small" rather than large. Many certified repair stations are small facilities. General aviation airports in this country have one or more mechanics who operate a small shop maintaining small aircraft. Some of these shops, that typically employ five to ten people, have repair station status. But, do they really need it?
So, it may make good sense to give up your certification, particularly if the new changes come into effect.
Turning in your repair station ticket would do several things. First, FAA surveillance would be reduced. There would be no need to maintain a repair station manual with inspection and training requirements. No need to designate a chief inspector. Possibly reduced insurance premiums. A smaller administrative burden. You can probably think of a few more benefits. What is the downside you ask? Well, you do lose a little prestige. Some customers may put more stock in a shop that was "certified." In today's business however, many don't think this is an important element. And, as mentioned above, you will have to revert to the 337 form for any major repairs or alterations. If there are any other downsides, I would like to hear about them.
Suggested Solutions
There have been many suggestions made but the most useful one seems to be a two-tier repair station setup. That is modifying the so called "tough" new rules and divide the shops into large and small stations. Old rules can apply to small shops and new rules to big outfits.
The reason being that the big outfits can afford the increased administrative load. As suggested, the dividing line could be turbine equipment and piston, or more logically, gross weight and horsepower (thrust) of the aircraft. The idea would be to separate the big guys from the little guys. The bottom line here is that a one-sized rule can't fit all the shops.
The Threat
Something has to be done in order to keep all the shops we have now. The result of the recently proposed rules will be less surveillance, fewer audits of work performed, perhaps many workers and repairmen out of work — all in the name of keeping pace with growth? Go figure.
Pick up a copy of the proposed new rule and read it. If you have access to the Internet, you can find information on the new proposed rule at http://www.nata-online.org/ part_145_rewrite_table_of_content. htm. Also, you can visit AMT magazine's Web site at www.amtonline. com for links to the Federal Register.
Although the official time to respond to the NPRM has passed (Notice of Proposed Rulemaking) you can still send in your two cents anytime.
Teledyne
Continuous Flow Fuel Injection Setup
by Teledyne Continental Motors
November 1999
According to Teledyne Continental Motors, operational verification of the TCM fuel injection system is required: at engine installation, during 100-hour or annual inspection, whenever a system component is replaced or adjusted, or when environmental changes occur.
Unfortunately, this requirement for setting up Continental engines for optimum performance is not always adhered to. Lack of understanding of the setup requirements can lead to technicians and operators accepting marginal perfor mance of their engines.
What many don't realize is that following proper setup procedures as outlined by the manufacturer and by TCM Service Information Directive SID-97-3 can make a world of difference in performance of an engine.
Following is a review of the procedure required by TCM to properly set up the Continuous flow fuel injection system:
Before beginning the fuel injection setup procedure, be sure to have appropriate aircraft and engine manuals to accomplish the procedure. Begin with the aircraft manufacturer's instructions and if they don't provide any data for adjusting the fuel injection system, refer to the appropriate Continental engine manual. Also, aircraft and engines that have been modified from their original type design must be maintained in accordance with the STC holders FAA- approved instructions.M
Make a copy of the Operational Test Form from TCM Service Information Directive SID-97-3. Record Idle and full RPM, metered and unmetered fuel pressure, fuel flow, manifold pressure, and idle mixture rise.
Pre-Setup requirements
First, verify the accuracy of the aircraft engine instruments, tachometer, manifold pressure gauge and fuel flow gage must be verified. For safety's sake, perform all work and make all adjustments with the engine stopped and the ignition and master switches in the OFF position.
Remove, inspect, and clean fuel system screens and filters in according to aircraft manufacturer's instructions. There are two instances where the fuel system must be flushed prior to continuing: 1) At engine installation, and 2) whenever fuel system components are replaced.
To flush the aircraft fuel lines in the event of an engine change, attach a length of hose to the fuel pump inlet hose and run the boost pump long enough to allow a minimum of one gallon of fuel to flow into a clean container. Inspect the fuel for contamination and continue to flush if any contamination is noted. If components in the fuel injection system are changed, associated fuel injection system lines must be flushed as well.
Next, verify proper routing and attachment of fuel injection system plumbing, verify proper installation of fuel pump vapor return line and check valve. Inspect the induction air filter for cleanliness and proper operation. Ensure proper operation of the alternate air source. Verify the throttle and mixture control and propeller control linkage operate properly, are rigged correctly, are lubricated, and are in good condition. Inspect the induction, exhaust, and fuel injection system for leaks, stains, chafing of lines, etc., and correct any problems before proceeding.
Connecting test equipment
Keep in mind that hose diameters on your test equipment can impact the readings you get while adjusting the fuel system. That's why it's important to be sure all fittings, T-connectors, and hoses are at least the same inside diameter as the pump outlet fitting. Extra length test hoses can also adversely affect the results. Following the aircraft manufacturers instructions, turn the fuel selector valve to the off position.
Loosen and remove the unmetered fuel supply hose from either the fuel pump outlet fitting or the fuel control inlet fitting, whichever is more accessible. Some aircraft have an unmetered fuel pressure fitting you can access at the fuel control screen.
Attach the unmetered fuel supply hose to the straight end of the T-connector and torque, then install and torque the swivel-T directly to the fuel pump outlet fitting or fuel control inlet fitting as applicable. Then, connect the line to the portatest unit or test gages. Be sure test hoses are sufficient length to be out of the way for engine runup.
Next, install the second swivel-T to measure metered fuel pressure. To accomplish this, disconnect the fuel inlet to the manifold valve fuel inlet fitting and iT-fitting. Then connect the portatest unit to the swivel-T. Torque as specified in TCM Service Information Directive SID-97-3 or most current revision.
Using the aircraft boost pump, pressurize the fuel system to bleed all air from the lines and hoses and check for leaks. Correct any problems before continuing. Also, be sure to allow sufficient time for fuel to drain from the entire induction system prior to starting the engine. Failure to comply could result in a hydraulic lock event.
Next, locate the idle speed stop screw on the throttle body and back it out two complete turns. During setup, idle speed rpm will be adjusted manually using the cockpit throttle control.
Before reinstalling the cowling, position test hoses so they are away from the exhaust system and are not damaged.
Performing the setup
With the test equipment connected, it's time to perform the setup.
Obtain the appropriate data for setup from the appropriate manuals.
Ensure the aircraft is secure with the brakes set, the wheels chocked, and start the engine. Focus your attention on the aircraft engine gages.
With the engine's oil pressure stabilized, advance the power to 1,500 to 1,800 rpm and wait for the oil temperatures to come up to normal operating temperatures. With the mixture full rich, reduce engine power to the specified idle rpm.
It is critical to accurately document all engine test parameters on the fuel system operational test form. This is the form SB 97-3 on which the manufacturer's specs were previously recorded.
Check the unmetered fuel pump pressure at idle rpm and record this reading on the operational test form. Next, check the idle fuel air mixture by slowly moving the mixture control from full rich to idle cutoff. A rise of 25 to 50 rpm should occur as the mixture control is slowly moved from full rich to idle cutoff. Return the mixture control to full rich and record the actual rpm rise on the operational test form.
With the mixture control full rich, advance the throttle to full power. On turbocharged engines, verify the manifold pressure is at the value specified by the aircraft manufacturer's maintenance manual. Record the rpm, metered fuel pressure, unmetered fuel pressure, and manifold pressure on the operational test form.
Using the operational test form, compare the actual readings with the required readings for the following areas:
- Idle fuel pressure
- Fuel/Air mixture rise
- Full power fuel pressures
- Fuel flow
- RPM install the
- Manifold pressure readings.
Boosting Your Knowlege of Turbocharging
Part II -Valves and Controllers
By Randy Knuteson
October 1999
Raytheon Aircraft's turbocharged Barons, Bonanzas, and Dukes depend on a Variable Absolute Pressure Controller. The VAPC has a bellows that extends into the upper deck air stream, sensing deck pressure and comparing it to a referenced absolute pressure. What makes this controller "variable" is that it is linked directly to the engine throttle. Through a system of cams and followers, it adjusts a moveable poppet seat and accordingly achieves the optimum deck pressure for a given throttle setting.
Continental's turbocharged engines rely on absolute, variable absolute, pressure ratio, dual, or slope controllers. Dual controllers are a hybrid combination of absolute controllers coupled with a rate or ratio controller. These units are housed in a common body and were most frequently used in the older Cessna 200-300-, and 400-series engines. TCM equipped Piper Malibus, the TSIO-550 equipped experimental Lancair IV, Piper's new PA-32 and Cessna's new 206's all incorporate yet another style of controller — the sloped controller. The sloped controller incorporates the functions of an absolute pressure controller and a differential controller in a single housing. Acting much like the density and differential arrangement in the Navajo, the sloped controller maintains the rated deck pressure at wide-open throttle, and a reduced deck pressure at part throttle, and looks for discrepancies in the differential between deck and manifold pressures. If either pressures rise above a pre-determined value for a given throttle setting, the sloped controller opens the exhaust bypass valve, thereby lowering compressor speed and output. However, unlike the density controller, it does not correct for temperature excursions.
Manual control systems
Not all turbocharging systems are automatically actuated. Instead, systems like those found in the Turbo Arrow and the Mooney 231 have a simple, built-in exhaust leak upstream of the turbo that allows a fraction of the exhaust gases to be dumped overboard and the remainder to be routed through the turbine. This "fixed" wastegate arrangement has also been successfully used in the Seneca II-IV. Other systems rely on manual inputs from the pilot. As power begins to decrease at higher altitudes with the throttle in the wide-open position, the pilot then incrementally adjusts the wastegate toward closure utilizing a separate vernier wastegate control cable. Or, as in the case of Cessna's Turbo Skylane and Piper's Turbo Saratoga, some installations have the wastegate linked directly to the throttle. As the wastegate valve closes, backpressure forces additional exhaust gases through the turbine wheel assembly and speeds up the compressor. This series of events results in an increase in engine manifold air pressure and a resulting increase in power. The pilot becomes the "controller" of these systems as he systematically monitors and regulates manifold pressures while ascending/descending in altitude. These simplified manual systems reduce both cost and complexity, albeit at the expense of increased pilot workload. This type of arrangement is often used in aftermarket turbo-normalizing kits.
Overshoot/Overboost
During the early stages in the development of these systems, problems arose with momentary overboost of as much as 6 to 10 inches of Hg during throttle advancement. To compensate, pilots were instructed to slowly advance the throttle to 32-in., wait for a few seconds, and then continue to the rated 35-in. of MAP. This surge in overboost was dramatically reduced to 2 to 3 inches of Hg with the development of the GTSIO-520 engines. By modifying the bore size of the wastegate actuator, designers found they could minimize the time it took for oil displacement and wastegate actuation.
Safeguards are required to prevent the overboosting of an aircraft engine. Overboosting occurs whenever maximum rated manifold pressures are exceeded. Properly adjusted controllers and proper throttle management serve to preclude the onset of overboost. However, despite these measures, inadvertent overboost still happens. An incidence of overboost can take place when the engine oil has not yet reached operating temps and the throttle is rapidly advanced for takeoff or when a pilot on short final quickly advances the throttle for a go-around and the prop governor lags or the wastegate sticks. Overboost can also occur at full throttle and below critical altitude when exhaust gases expelled are more than capable of driving an uncontrolled compressor to engine damaging speeds.
What differentiates between Overshoot and Overboost? Answer: Duration, or the length of time manifold pressures exceed their maximum rate. "Overshoot" occurs when manifold pressure increases above maximum rate by one or two inches, and then returns immediately to the maximum rated. Lycoming states that if overshoot does not exceed two inches and three seconds duration, it may be disregarded. TCM says, "If the amount of overboost is from three to six inches, the system should be checked immediately for necessary adjustments or replacement of the malfunctioning component." As defined by Textron Lycoming, "momentary overspeed" is "an increase of no more than ten percent of rated engine RPM for a period not exceeding three seconds."
A table for computing overspeed by engine model is included in Lycoming SB#369I. Stringent inspection criteria are specified for engines subjected to inadvertent overspeed. These procedures include a thorough examination of the cylinders, valves, guides, seats, counterweights, etc.
How do you spell relief?ÉP.R.V.
The Pilots' responsibility is to ensure that manifold pressures stay within their prescribed limits. But, humans are known to be fallible. With this in mind, thoughtful engineers sought to incorporate a secondary safety feature known as an absolute Pressure Relief Valve ("pop-off valve"). This valve is strategically positioned between the compressor outlet and the fuel injector/carburetor to prevent engine-damaging surges in manifold pressure. The valve head is normally held in the closed position by a spring/bellows combination. It is set to off-seat at approximately two inches above normally rated MAP. Continental Motors compares the operation of this spring-loaded valve to a safety valve in a steam boiler. As excessive pressure builds, the valve opens and stays open until the manifold pressure again falls within acceptable limits. The aneroid-style bellows allows the valve to "crack" open at a specified pressure regardless of the operational altitude of the engine.
In the Mooney 231 (TSIO-360-GB/LB), and the Piper Turbo Arrow and Senecas (TSIO-360-E/EB/F/FB/KB), the Absolute Pressure Relief Valve functions as a turbocharger control/boost limiter. These engines incorporate a "fixed" wastegate arrangement and do not rely on other servo control systems. In this design, the PRV is set to vent any compressor discharge pressures exceeding one inch Hg above rated manifold pressure. This "crack" pressure increases as a function of altitude, thereby maintaining a constant manifold pressure as relief flow past the valve head varies. This "modulating" valve successfully compensates for variations in compressor discharge air-flow.
Teledyne
Continuous Flow Fuel Injection Setup
continued...page 2 of 3
Fuel system adjustments
It's important to remember that adjustment may require multiple tries to stabilize the system at the desired pressures. One adjustment typically affects other parameters in the following manner:
- Adjustments to the high end have a significant impact on the low end pressures.
- Adjustments to the low end have a significant affect to the operating range.
The four adjustments that can be made to the fuel injection system are:
Now, since fuel system adjustments can only be made with the engine off, make only small incremental changes between measurements. If large adjustments are made arbitrarily, system balance may be lost and this may require you have the factory recalibrate the components on a fuel bench.
If the fuel pump pressure at idle rpm was not specifications as per the engine operational test form, then the relief valve will need to be adjusted.
Locate the relief valve adjustment on the rear of the fuel pump. Loosen and hold the lock nut and turn the valve clockwise to increase pump pressure, or counterclockwise to decrease pump pressure.
Once you've completed the adjustment, start the engine and again let the oil temperature and pressure stabilize at 1,500 to 1,800 rpm.
Now, retard the throttle to the specified idle rpm. Monitoring the tachometer, slowly move the mixture control from full rich to idle cutoff to check the idle fuel/air mixture adjustment. A 25 to 50 rpm rise should occur. If the rpm rise is greater than 50 rpm, the fuel mixture is too rich, if the rpm rise is less than 25 rpm, the fuel mixture is too lean.
Adjusting the pump pressure and idle mixture requires that the technician achieve a balance between idle 28 Recip Technology pump pressure and idle mixture. So everytime one adjustment is made, the other has to be checked to see how it was affected by the change. Additionally, it can't be emphasized enough that if the engine is not brought up to normal engine temperatures and pressures, and if the engine is not cleared by running to 1,500 to 1,800 rpm between adjust ments, it will not be possible to properly adjust the system.
To adjust the idle fuel air mixture on engines using the link rod between the throttle and fuel control unit, turn the 3/8-inch lock nut clockwise to enrich the mixture or counterclockwise to lean the mixture.
The GTSIO-520 idle adjustment requires clockwise rotation to lean the mixture and counterclockwise rotation to enrich the mixture.
Some engines (IO-240, IO-360, IO-550-G, TSIO-360, TSIO-520-D, DB, UB, BE, and TSIO-550-B, C, and E) use a combination throttle fuel control assembly with incorporated fuel control unit. The idle fuel air mixture adjustment is located in a recess in the fuel control unit cover attached to the side of the throttle body. Clockwise rotation of this adjustment screw will lean the mixture setting, counter-clockwise rotation will enrich the mixture setting.
Since adjustments are made with the engines turned off, you may need to repeat the adjustments several times before you get it right.
You have now completed procedures for setting the fuel injection system at idle speed. Next, you need to set the fuel injection system at full power.
For turbocharged engines, the idle performance adjustments are identical to naturally aspirated engines; however, if you have a turbocharged model, you will need to refer to the turbocharged parameters to adjust full power settings.
Fullpower settings
Start the engine again and bring it to normal operating temperatures. Engine fuel flows and pressures vary with engine rpm. With the mixture at full rich and the propeller control to full increase rpm, slowly advance the throttle to full rated power for the engine. Allow the engine to stabilize for at least least 15 to 30 seconds before taking any readings.
After the engine stabilizes, take the following readings:
- Fullpower metered and unmetered pressure
- Fuel flow
- Manifold pressure
- RPM
Retard the throttle to 1,000 rpm and compare the actual readings with the required specifications.
If pressures and flows are within the specified limits, no further adjustments have to be made and all test equipment can be removed and the aircraft returned to service.
If the parameters are not within specified values, further adjustments are necessary.
Turn off and disarm the engine, then locate the adjustable orifice on the fuel pump's vapor separator body. On all fuel pumps, turning the adjustable orifice clockwise will increase unmetered fuel pump pressure; turning it counterclockwise will decrease the unmetered fuel pump pressure. The orifice uses either a 5/32 allen adjustment or a slotted screw adjustment, depending on when the engine was manufactured.
With this adjustment, you are changing the unmetered fuel pressure by turning the adjustable orifice at the engine driven fuel pump to attain the correct metered fuel pressure and flow.
Naturally aspirated engines that use an altitude compensating fuel pump allows fuel pump pressure to vary with atmospheric pressure. The aneroid is preset at the factory or at overhaul. It should only be adjusted following a flight evaluation in accordance with TCM Service Information Directive SID-97-3 or the latest revision.
Perform a complete system operational test and verify that all fuel system parameters are within the specified values for the aircraft and engine.
Turbocharged engine adjustments
Although the approach is basically the same for turbocharged engines, the process is different enough that it warrants a careful review of both the aircraft manufacturer's instructions and Service Information Directive 97-3.
Remember that you may have to disconnect the fuel pressure regulator if installed and reattach the lines, minus the regulator for adjustment.
Compensating
One mistake that is often made by technicians is compensating for the pressures specified in the bulletins due to either a removed fuel regulator or the engine not able to make the rated rpm.
If your turbocharged engine model incorporated a fuel pressure regulator, you adjusted the full-power parameters five percent higher than the published specifications for the engine. Remember to reinstall the fuel pressure regulator removing plugs and caps and reinstalling the line on the regulator.
The other condition that you may have to compensate for is if the engine is not making full rated power. For instance, if the specifications call for the unmetered pump pressure to be 28.3 to 29.8 psi at 2,625 rpm and the engine will only turn at 2,600 rpm, the unmetered pressure will have to be adjusteAMd down by a factor as listed in TCM's Compensation Table for Static Ground Setup. This table can be found on Page 9 of SID97-3.
One final note; It's not uncommon to have to repeat these procedures several times prior to achieving desired results. So don't get discouraged if the procedure doesn't produce the desired results the first time. The most experienced technicians may have to repeat the procedure until it's correct.
Teledyne
Continuous Flow Fuel Injection Setup
by Teledyne Continental Motors
November 1999
continued...page 3 of 3
Teledyne Continental Motors Update
Teledyne Continental Motors has proven to be one of the leaders in innovation with respect to aviation piston engines. With the introduction of fuel injection and turbocharging in the '60s to a liquid cooled engine in 1986, the company is indisputably in the forefront of piston engine development. And, the last couple of years have been no exception.
R ecent developments at TCM include a Jet-A fuel burning engine developed as part of the NASA General Aviation Propulsion (GAP) program
The engine is designed with cast case halves with integral cylinder assemblies.
The idea is that little or no maintenance will be required until TBO, which is projected to be around 3,000 hours.
The company says the fact the engine burns JetA fuel will be appealing to operators around the world as Jet-A is readily available worldwide. A prototype is currently in test at the TCM factory.
FADEC
The company has also launched into program to upgrade existing technology to electronically controlled engines.
The FADEC (Full Authority Digital Engine Control) design has come a long way with help from Teledyne's sister company, Aerosance, Inc.
With the Aerosance FADEC, each engine cylinder is controlled by a dedicated microprocessor and is backed by a second microprocessor. Magnetos are replaced with MPC (Master Power Control) units which control fuel and ignition. Each unit contains the igniter coils (primary and secondary) and fires two cylinders. Fuel injectors are redesigned and control fuel pressure and distribution independently.
With FAA certification expected late this year, sales of the Aerosance system could begin as early as January of 2000.
Updated machining and manufacturing
Additionally, TCM has spent the last several years updating their factory to reduce manufacturing costs and provide ontime support and quick turnaround for their customers.
President Brian Lewis says the company has invested millions since 1986 and reduced the space required to manufacture and rebuild engines by 75 percent.
The result is that TCM's manufacturing is more consistent and timely. Computerized tracking process also allow the company to track individual orders through the factory.
TCM Link
Although the company has had a computerbased system for communicating and providing information to its customers and TCM Support centers, the company has now converted this system for use on the World Wide Web and has added many new features.
Access to this page allows TCM-approved service centers to gain access to critical maintenance information, track information on specific engines by serial number, and request quotes for TCM parts from TCM approved distributors.
Training School
TCM has now moved its school to the factory in Mobile, AL, with a new emphasis on complete and thorough training of technicians who purchase the one week course. The course includes all the maintenance manuals and applicable documents you'll ever need to adjust and maintain the engine. In addition, the attendee is offered the opportunity to adjust critical components in the fuel injection system on the test bench to gain a true understanding of the affects of the different adjustments.
The course is well worth the money. For more information or to reserve a space in the class, call Don Fitzgerald, Jr. at (334) 436-8134 or e-mail to
Window Repair Basics A clear view of window maintenance requirements
July 1999
Typically, crazing is repairable if addressed early, and if adequate thickness is present to maintain the required minimum thickness of the window.
Crazing that has progressed to cracks cannot be repaired. Cupery explains, "When you see multiple crazing marks and they are all chained or interconnected, they are then classified as cracks and cannot typically be repaired."
According to Cupery, crazing was not always a problem in the industry. "Before 1980, crazing was not that bad a problem. Unfortunately, in 1982, the volcano on Mt. El Chicone pumped megatons of sulfur dioxide gas into the atmosphere. This gas quickly circled the earth and was present around the globe. Since then, other volcanic events, such as Mt. Penetubo in the Philippines have added to the problem. This gas mixes readily with moisture in the atmosphere and, at certain altitudes, becomes sulfuric acid. The resulting acid etches the windows and is primarily responsible for most of the crazing we see today. Note that all volcanoes have not contributed to the problem. Some, such as Mt. St. Helen, pumped out lots of ash, but did not produce sulfur."
Cleaning can result in crazing also, in addition to a host of other damage Ñ essentially, chemicals and heat cause crazing. With this in mind, what you use to clean windows can be responsible for the crazing of your windows.
Adhesive tape used in this Challenger window can cause what is called "corn-flaking."First of all, don't use glass cleaners or cleaners that contain ammonia or other harsh chemicals. Use a cleaner that is designed for acrylic. In addition to harsh chemicals, glass cleaners also end up charging the window with static electricity, which means that the window will attract dust, dirt, and other airborne particles. A true plastics cleaner should have an anti-static agent built in. So, use glass cleaners on glass only and use plastic cleaners on acrylic.
Other items such as adhesives used during masking and painting operations can also result in crazing if the tape residue is not removed after the paint job.
Methyl Ethyl Ketone (MEK) is particularly harmful to acrylic. In fact, MEK and acrylic windows just don't go together. It will be only a matter of seconds after MEK is applied before it attacks the acrylic and causes crazing. "We have had to repair numerous windows where maintenance personnel used MEK to remove paint overspray or to clean the windows," says Cupery.
Another source of damage can be from using the wrong sealant. "We have found several cases of this, and typically, if one window exhibits this type of damage, they all will have it," warns Cupery. "In fact, on any given aircraft, damage that is found on one window should lead you to suspect the same types of damage to the remainder of the windows. Don't let anyone talk you into using alternate sealants other than those approved by the manufacturer. Some Ôquick curing' sealants that allow your aircraft to return to service quicker have accelerators in the sealants that can attack the acrylic. Also, RTV 732 has Glacial Acetic Acid in it that will attack the acrylic Ñ so don't use it to make repairs. Additionally, on the other end of the spectrum, there are products that are alkaline in nature (with a Ph level above 7) and can also be caustic and cause crazing."
Painting operations offer their own set of particular problems related to windows. Cupery continues, "We've done many windows that have razor damage from cutting templates to cover the windows during painting. In the early 80s, paint shops were unaware of the potential damage that could be done to windows. They would cut right into the acrylic, which eventually resulted in many window failures. During one year early in the 1980s, we repaired over 300 Hawker aircraft alone that had razor damage."
Fortunately, paint shops have become more aware of this problem. The number of windows that require razor damage repair is diminishing over time, as the industry becomes more educated.
Another problem is related to chemicals and strippers. Most shops know not to let strippers come in contact with the windows, but in an effort to remove paint right up to the edge of the window; they unwittingly allow the stripper to migrate into and under the window flange. The stripper eventually works on the acrylic, weakening the window and causing crazing.
A special prism is required to be used with glycol (as a coupling agent) to view damage to the outer edges of the windowThere are various ways to avoid this, including properly masking the space between the skin and the windows so as not to allow chemicals and strippers to enter, or removing the windows and installing painting blanks. Either way, the cost of taking these extra steps is far less than having to buy a new set of windows.
Another common cause of crazing is hot patches on windshields. The heat eventually forms fine crazing that begins to distort the pilot's view. Fortunately, up to 80 percent of the crazing that happens to hot patches (heated windshields) can be repaired.
Shelling/Scaling
Shelling is the in-plane cracking of the acrylic panel. The damage has a shiny "oyster shell" appearance. Typically this damage will be evident around bolt holes and in the outer radius areas of the window. It may be caused by razor cuts, stress risers on the radius, or over torquing of bolts during installation and is also referred to as scaling. This is a condition that can cause structural failure of the window and must be removed from service.
"Shelling reflects back at you like the inside of an oyster shell when you shine light directly onto it," Cupery explains. Shelling is difficult, if not impossible to repair and the window usually must be removed from service."
Debonding
Debonding is a separation of two or more surfaces that had been bonded together through the use of specific bonding materials, which begin to deteriorate. This is typically evidenced by the formation of moisture and fogging in the windows due to moist air bypassing the desiccant system. Many windows use a desiccant system to remove moisture from the air that is circulated between the windowpanes so as to prevent fogging. This is not a structural problem and is repairable; however, moisture can cause corrosion to occur in some windows, e.g. Hawker D/V windows. Cupery says "Older windows were also more prone to this with some of the Challenger cabin windows having a particular problem of the adhesive tape drying out and flaking into the window. At the time, it was known as a Ôcorn flaking' problem because the flaking cellophane resembled corn flakes. The problem has since been resolved after developing a new seal that doesn't exhibit the same problem. Debonding of double pane windows is evident as the window will fog over and moisture will build inside the window.
Once debonding begins, it quickly gets worse. What happens is moisture builds in the bottom of the window and between the panes. As the aircraft goes to altitude, this moisture freezes and spreads the panes even further apart Ñ causing further debonding.
Delamination appears as a milky white area that doesn't reflect back. Delamination
Delamination is the separation of the acrylic or glass layer from the polyvinyl sheeting to which it is laminated. Causes are usually stresses from normal airframe torquing or age and UV degradation. This is not considered to be a structural problem and is actually a built-in form of stress relief, yet it may grow to obstruct vision. Repairs, in many are not cost effective, approved, or long lasting. However, we have had some very good success at relaminating some King Air, Soccata, and Falcon windshields. This procedure is successful only if the delamination is clear and there is no moisture ingression. Once moisture is present, which is evidenced by a milky color, the window cannot be relaminated.
Delamination can be distinguished from shelling because it doesn't reflect back at you when you shine a light on it. Instead, it just appears as clear bubbles, dull flat or white discolored areas. Manufactures have limits for how large an area is allowed. If the area of delamination is within manufacturer's limits, you should monitor the area for growth by marking the edge of the damage with a grease pencil on the inner surface of the window. In any case, if the pilot's vision is affected, the window should be replaced.
Erosion
Another thing that can take a toll on windows and cause unexpected stress risers is simple erosion. One example of this is on the Falcon 10 DV (side cockpit) windows. The DV window on this particular aircraft protrudes out into the airstream and airborne dust particles and rain slowly erode the leading edge of the window. We repair these by beveling, within certain manufacturer's limitations, the leading edge so that it is more aerodynamic. This erosion is unacceptable and must be repaired/removed if you notice it or shelling will occur.
Can your windows be repaired?
According to Cupery, "Different aircraft windows have different repairability levels. Repairability will depend on how much material exist beyond minimum thickness limitations. Some aircraft manufacturers try to reduce weight and engineer the aircraft with the windows already at minimum thickness. This does not allow for repairability. Others add material for repairability and safety."
There are also windows that are located in areas that require more attention then others. An example is the Jetstream 31. On this aircraft, there are a lot of problems related to heat and chemicals on the center windshield.
The interior detail and complexity of the aircraft will often determine whether it is cost effective to take the aircraft window out or not. Often, most of the damage is on the outside, so it is quite cost effective to just leave the window in stalled on the aircraft for repair.
Cupery stresses that you can't address damage soon enough. Letting damage progress can mean that repair costs go up exponentially. "Often, we take a window back to the shop after quoting a price for minor repair and then have to inform the operator that they've got more damage than expected. This results in more time and costs to remove the damage."
He continues, "But keep in mind that when we repair a window in the shop, we address much more than just the surface you're looking through. We spend a great deal of time uncovering, inspecting, and repairing the areas of the window that are hidden under the edges of the window openings. This may mean re-radiusing edges and dressing boltholes, and annealing windows to remove stresses when called out by the manufacturer."
Debonding
Debonding is a separation of two or more surfaces that had been bonded together through the use of specific bonding materials, which begin to deteriorate. This is typically evidenced by the formation of moisture and fogging in the windows due to moist air bypassing the desiccant system. Many windows use a desiccant system to remove moisture from the air that is circulated between the windowpanes so as to prevent fogging. This is not a structural problem and is repairable; however, moisture can cause corrosion to occur in some windows, e.g. Hawker D/V windows. Cupery says "Older windows were also more prone to this with some of the Challenger cabin windows having a particular problem of the adhesive tape drying out and flaking into the window. At the time, it was known as a Ôcorn flaking' problem because the flaking cellophane resembled corn flakes. The problem has since been resolved after developing a new seal that doesn't exhibit the same problem. Debonding of double pane windows is evident as the window will fog over and moisture will build inside the window.
Once debonding begins, it quickly gets worse. What happens is moisture builds in the bottom of the window and between the panes. As the aircraft goes to altitude, this moisture freezes and spreads the panes even further apart Ñ causing further debonding.
Delamination
Delamination is the separation of the acrylic or glass layer from the polyvinyl sheeting to which it is laminated. Causes are usually stresses from normal airframe torquing or age and UV degradation. This is not considered to be a structural problem and is actually a built-in form of stress relief, yet it may grow to obstruct vision. Repairs, in many are not cost effective, approved, or long lasting. However, we have had some very good success at relaminating some King Air, Soccata, and Falcon windshields. This procedure is successful only if the delamination is clear and there is no moisture ingression. Once moisture is present, which is evidenced by a milky color, the window cannot be relaminated.
Delamination can be distinguished from shelling because it doesn't reflect back at you when you shine a light on it. Instead, it just appears as clear bubbles, dull flat or white discolored areas. Manufactures have limits for how large an area is allowed. If the area of delamination is within manufacturer's limits, you should monitor the area for growth by marking the edge of the damage with a grease pencil on the inner surface of the window. In any case, if the pilot's vision is affected, the window should be replaced.
Erosion
Another thing that can take a toll on windows and cause unexpected stress risers is simple erosion. One example of this is on the Falcon 10 DV (side cockpit) windows. The DV window on this particular aircraft protrudes out into the airstream and airborne dust particles and rain slowly erode the leading edge of the window. We repair these by beveling, within certain manufacturer's limitations, the leading edge so that it is more aerodynamic. This erosion is unacceptable and must be repaired/removed if you notice it or shelling will occur.
Can your windows be repaired?
According to Cupery, "Different aircraft windows have different repairability levels. Repairability will depend on how much material exist beyond minimum thickness limitations. Some aircraft manufacturers try to reduce weight and engineer the aircraft with the windows already at minimum thickness. This does not allow for repairability. Others add material for repairability and safety."
There are also windows that are located in areas that require more attention then others. An example is the Jetstream 31. On this aircraft, there are a lot of problems related to heat and chemicals on the center windshield.
The interior detail and complexity of the aircraft will often determine whether it is cost effective to take the aircraft window out or not. Often, most of the damage is on the outside, so it is quite cost effective to just leave the window in stalled on the aircraft for repair.
Cupery stresses that you can't address damage soon enough. Letting damage progress can mean that repair costs go up exponentially. "Often, we take a window back to the shop after quoting a price for minor repair and then have to inform the operator that they've got more damage than expected. This results in more time and costs to remove the damage."
He continues, "But keep in mind that when we repair a window in the shop, we address much more than just the surface you're looking through. We spend a great deal of time uncovering, inspecting, and repairing the areas of the window that are hidden under the edges of the window openings. This may mean re-radiusing edges and dressing boltholes, and annealing windows to remove stresses when called out by the manufacturer." 
Source: https://www.aviationpros.com/home/article/10388951/teledyne-continuous-flow-fuel-injection
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