I recently got a letter from Verl Thompson, who is restoring a 1960s Piper TriPacer. He points out that one of the weak points of this aircraft is the jack screw in the tail that controls the elevator trim. The jack screw is about 8 inches long and a half inch in diameter. There is a pulley that runs up and down on the shaft, which is turned via a cable and crank above the pilot’s head. Unfortunately, everything has to be properly adjusted or the cable can slip while in the air since the normal slip stream of air does resist the movement of the elevator.
I recently got a letter from Verl Thompson, who is restoring a 1960s Piper TriPacer. He points out that one of the weak points of this aircraft is the jack screw in the tail that controls the elevator trim. The jack screw is about 8 inches long and a half inch in diameter. There is a pulley that runs up and down on the shaft, which is turned via a cable and crank above the pilot’s head. Unfortunately, everything has to be properly adjusted or the cable can slip while in the air since the normal slip stream of air does resist the movement of the elevator.
“”What lubricant would you recommend for the new jack shaft I am installing?”” he asks. “”I have had suggestions for everything from WD-40 to axle grease.””
The only legal and correct answer is to look in the aircraft manual and find the specified product called for in that application. I do not know what year your aircraft is, so I can not give you a hard recommendation. I did, however, check with a friend who has an earlier TriPacer. His manual lists Mil-G-23827 grease as recommended for the jack screw and a Mil-L-7870 oil for the shaft. It also states that no lubricant should be put on the cable. A Mil-G-23827 grease like Aeroshell Grease 7 should be available from a lubricant or aviation supply house. It is also available over the Internet in small quantities.
My main concern about this letter was the “”I have had suggestions for everything from WD-40 to axle grease”” statement. One of the problems in the lubricants business is that almost every one is an expert (I have worked in the fuel and lubricant business for more than 35 years and I consider myself an expert-in-training). If a person tries WD-40 on an aircraft jack screw and the plane does not fall out of the sky, he becomes an expert. Never mind that the factory spent literally thousands of hours evaluating different products to determine a recommendation that will work under all types of conditions. People will listen to this one person who, under very limited conditions, may have just lucked out and it worked.
WD-40 is primarily penetrating oil, which gives a small amount of temporary lubrication when first applied. If sprayed on parts it tends to get on parts it shouldn’t and not always carry the desired load. The axle grease recommendation is not a real bad recommendation under normal conditions. However, if one flies into a cold climate, the axle grease may become too stiff, which will greatly increase the torque needed to turn the jack screw, or actually cause it not to function at all. Then there is the concern about product compatibility.
The bottom line is to use the products that are recommended by the factory. It is not only required by FAA regulations, it may just save your life.
Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at [email protected].
