Aviation From My Rocking Chair: Aerobatics: Special Maintenance and Housekeeping

We all have attended airshows and marveled at the aerobatic performers flying Extras, Pitts, Cap’s, and even some Stearmans. Some of these aircraft are stressed to +/- 10g and often near that limit during a routine. As you can imagine, there is some specialized maintenance that must be done in order to keep these machines airborne. Let’s go over some of these requirements, bearing in mind that most of these checks could be applicable to any aircraft.

(1) Keep it clean inside (especially) and out. If you drill inside, clean the chips. If you drop hardware, retrieve it. If your feet are dirty, vacuum the floorboards and anything else that may hold dirt. Be sure to count your tools before and after the job…the number must match! I once did an annual on a clipped-wing Cub which belonged to an airline pilot. The airline wouldn’t let him roll his DC-9 so he rolled the Cub every chance he got.

As I was removing a belly inspection plate to check the aft stick linkage, a 6 inch screwdriver fell out and hit me in the forehead (probably the safest place to hit me)! The Captain’s comment? “Oh, that’s where that thing went. I lost it months ago.” I still shudder to think what would have happened if that screwdriver had fallen into the control stick assembly while the Cub was blue side down. Keeping everything clean allows you to see fluid leaks early, inspect structure more easily, and helps your engine operate better due to more efficient cooling (clean cylinder fins.)

(2) An aerobatic aircraft may go from +10g to -10g in 2 seconds. Airspeed can vary from +200 kts to -50kts. In flat spins and snap rolls, the pitch and/or yaw change can exceed 360 degrees a second. This type of performance puts a tremendous load on the pilot, engine airframe, and especially the propeller. To make matters worse, a hollow engine crankshaft is required for a constant-speed prop, making it much weaker than a solid crank. Any suspicious oil leak in the prop hub area must be investigated immediately. Most propeller manufacturers recommend overhaul twice as often if the prop is used for aerobatics. Most engine manufacturers also suggest the engine TBO be halved for an aerobatic engine.

(3) Engine mounts take a lot of abuse. Check them often for wear and damage. It might be something to add to the preflight.

(4) The oil in an aerobatic engine does not sit in the sump, and neither does the junk that gets in it. Oil changes are at 25hrs, and an Oil analysis program is a must.

(5) Check for security of components and wiring. It is not unusual to find wiring bundles chaffing something several inches away during high G maneuvers.

(6) Flight control attach points, control rod ends and bellcranks. All are subject to increased wear and must be checked often. Figure … if it moves, it must continue to move, and if it is supposed to be rigid, just make sure it stays that way.

On a personal note, if I don’t get on the stick, my grand kids will need to finish my projects. With your kind permission, I’d like to ask for help from time to time. Thank you and I appreciate you. As always, thank you for your support. Writing this column has been the best education I could ever get and I don’t plan to stop anytime soon. Rick Vaux TC4130