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Learjet Flap Tracks: Perma Silk G Coating

September 2009

Recently there have been some questions raised about the lubrication requirements of the flap tracks on the Learjet 40 series aircraft. I will try to clear up some of the issues raised by technicians.


On later serial number 40 series aircraft, 45-408 & subsequent, 45-2129 & subsequent and earlier serial numbers modified per Service Bulletin 45-27-31 or 40-27-16, the flap tracks will have a coating of Perma Silk G. This is the only lubrication required or authorized.

On earlier aircraft that DO NOT have the Perma Silk G coating installed per the above referenced Service Bulletin, the flaps will have to be removed for the A4 (300 hour) inspection for the visual inspection of the flap nose rollers, flap tracks, flap nose roller tracks and flap main track rollers. This will result in an additional eight hours of labor cost for the flap R&R.

If your aircraft does have the Perma Silk G installed, the above mentioned items are visually inspected on the B4 (600 hour) inspection with the flaps installed. The Perma Silk G is then reapplied as necessary.


In accordance with Chapter 12 of the maintenance manual (MM-104 or MM-105) when Perma Silk G is not installed, a wide temperature general purpose grease that meets MIL-PRF-81322, such as Mobil 28 and AeroShell 22, is to be used for lubrication. For best results, the old grease needs to be cleaned off thoroughly and a very light coat of the new grease applied. As a standard shop practice, always check with the manufacturer for compatibility of greases before mixing. Some operators in the past have received permission from Learjet to use a MolyKote 312 or 3402C for aircraft that have not applied the Perma Silk G coating. As always, these are on a case-by-case basis and must be authorized through Learjet Engineering. As previously stated, if the aircraft does have the Perma Silk G coating, it is then reapplied as necessary and is the only lubrication required or authorized.


On the earlier aircraft without the Perma Silk G we’ve noticed during the inspection of the rollers, very few are not flat-spotted to some degree. Most of the failed rollers are due to improper cleaning or lubrication. The importance of removing the old grease that contains contaminants cannot be understated. Just as important is to properly apply the new grease. From our experience, over-applying the new grease can actually trap more dirt and contaminants. Currently the cost to replace a roller ranges by part number from $584 to $1167 each. With a total of 30 rollers that is a potential cost of almost $23,000 for just the rollers. Labor not included.


There have been reports finding varying degrees of surface rust developing on flap tracks. This has been very light and removable with Scotch Brite. These reports also show a less than expected wearing of the Perma Silk G with flaking of the coating. As stated earlier, properly preparing the surface is very important. Even with meticulous preparation of the surface, we are still seeing less than the expected wear characteristics of the Perma Silk G.

Duncan Aviation is working closely with Learjet Engineering and through the Learjet NBAA Tech Committee on solutions to these issues along with our operators.