ATA 32

Optimum Brake Service Life
Several of our customers have experienced brake part failures with resultant main wheel damage. I would like every operator to read the Aircraft Braking Systems Service Letter IAI 1124-SL-6 operating recommendations and see if you can lessen these incidences.

Additional Suggestions:

• Use the full runway – Several operators use the full runway length during landing, which allows engine cooling and requires less brake usage, resulting in less heat and wear. Midway back, they are able to shut down one engine so taxi speeds are lower.
• Excessive wear – Another possible factor is allowing the brakes to become worn past the max limit in an attempt to save money, I guess. We usually see excessive overhaul bill-back charges as a result IF the brake lives through it.
• Thrust Reversers – One simple step to extend the life of brakes is to use the reversers. On aircraft that we have maintained for customers who really use the T/Rs, we don’t see any more loose rivets, cracks, worn bearings, etc. in the empennage areas than customers who don’t use the T/Rs. If you look at the costs to perform the 5,000-hour T/R inspection (approximately $18K) versus the costs for ONE set of brakes ($14K), it seems the use of reversers is one of the best ways to save money. Think about that the next time you make a first turnoff landing.

ATA 34

RVSM Skin Mapping Reqiurements
Now that quite a few of our customers have had Reduced Vertical Separation Minimums (RVSM) approved aircraft for two years, several are going through the recertification process. Sticker shock seems to be the universal response when the invoice gets to them. I have tried to explain the process to them, but the following information from AeroMech can explain all the why’s and how’s of the requirements.

At some point in the very near future, AeroMech will have a place on our Web site (www.aeromechinc.com) that explains all the details of why, when, how the skin waviness/air data system interconnects, and why things work out the way they do. In the meantime, let’s start with this explanation. “Basically, all airplanes are different – even airplane derivatives can be different enough to warrant a different outcome for RVSM airworthiness compliance. Some airframes we’ve certified require no waviness measurements whatsoever, some require initial only, and some require repeated measurements. It depends on the mean altimetry system error (ASE) of the entire fleet, the variation of the ASE across the fleet (i.e., the accuracy of one Westwind airframe compared to another), and the allowable error allocation for each of the air data system components as calculated in the error budget. If every airplane had the same component error budget, you would expect the same skin contour requirements. The minute you’ve changed the airframe configuration (i.e., “shape” or engine configuration) or the avionics, or even the flight profiles, you can be assured you will have a different error budget (different skin waviness requirements). We had an advantage on the Westwind program because we worked with IAI on all of their production airplanes, and were able to talk with their engineers and gather data regarding the build and performance of the Westwind. Specifically, we know there is airframe variation within the fleet, and this variation contributes to variation in ASE. Our flight test results confirmed this as well. Larger variation generally means tighter controls on skin contour. On the avionics side, the ISS units are capable of a single curve only. If we have angle of attack capability, or multiple Static Source Error Correction (SSEC) profiles, that too would help with increasing the allowance for skin waviness, because the overall system could be even more accurate, and can, in fact, reduce the variation of ASE within the fleet. We found we could predict the variation in the Westwind fleet to within about 10 feet, using CFD techniques. This allows us to accurately calculate the error budget for skin waviness. Our fleet sampling showed that there is a likelihood that Westwinds exist that could fail the error budget and skin waviness. Unfortunately, if one airplane fails and is not caught, you run the risk that the entire fleet could be grounded. You can’t make any mistakes on even a single airplane. Due to the accuracy of our methods, we can easily assess each airplane and ensure they will pass RVSM requirements. The FAA has granted us approval authority for our techniques.
For continued airworthiness, there is very little room in the budget left to allow for skin contour drift over time. Drift characteristics are known for avionics components, but not for the skins, and we’ve seen everything over the years regarding the skins (having done over 50 Group certifications). We’ve seen airplane skins change every year for four years, then change little after that. Some airframe skins are always changing. Some never change over time in service. Therefore, it’s critical to know exactly what characteristic each airframe has and develop continued airworthiness requirements based on what you know about drift, and based on how much room you have in the error budget to account for it. These days everyone needs to be very keen on this, particularly with the renewed interest in RVSM continued airworthiness practices expressed by the FAA and JAA.
Hopefully, after gathering data on the fleet after 24 months in service, we will find that skin waviness didn’t change much (as a fleet), and therefore, we can possibly eliminate the need to do continued measurements. This is very airplane-dependent – some airplanes require 48-month checks before they can stop doing skin waviness, some 24 months, and some never. On some airplanes the error budget is so huge for skin waviness that measurements aren’t even required. That again depends on the airframe itself (how fast it flies, what avionics system it has with what SSEC capability, etc.). There are certain airframe types with certain types of pitot-static systems for which skin waviness measurement is not required; however, it is uncommon (we’ve seen perhaps 3-4 Groups out of the 50 we’ve done). Further, only one set of skin waviness measurements (taken initially) are great, but they don’t tell you anything about what’s going to happen to that airplane over time in service. We have a huge database of nearly every type of common commercial/business jet airframe that shows that even airplanes within a specific Group will change differently over time, depending on how wavy their skins are in the first place. It gets pretty complicated, but having 5,000 airplanes’ worth of skin waviness data really tells a lot about what goes on with these airplanes over time. Further, we have flight test and GMU/HMU correlations on every airframe type, and we know how well our continued airworthiness programs are working out.