Aerobic Aircraft

April 1, 2002
Some best practice tips on maintaining high performance aircraft

By Joe Escobar

There it was glistening in the sun. It was an Extra 300L sitting on display at the EAA fly-in in Oshkosh last year. It was my first up close look at an aerobatic aircraft. The aircraft seemed to scream "I mean business!"

An aerobatic aircraft like the one that was before me is to a skilled pilot what a paint brush is to an artist. Many of today's top airshow pilots such as Patty Wagstaff and Mike Mancuso have chosen the Extra 300L as their aircraft for creating exciting performances for airshow spectators. But extreme aerobatic maneuvers are demanding on an aircraft. In order to meet these demands, the Extra 300L was designed to be a stout and agile aircraft. It has a +/- 10G capability. No detail is left out from the incorporation composites in most of the structure to the very tight tolerances in the flight controls.

Its Lycoming AEIO 540 engine is specially designed for the unusual attitudes encountered during aerobatic flight. Attempting to perform these maneuvers in a normal recip. engine could result in engine stoppage from either fuel or oil starvation.

Fuel supply

The engine uses a fuel injection system, measuring air flow and metering fuel to the inlet ports of each cylinder. Float-type carburetors are not used because inverted flight for more than a few seconds would cause it to stop metering fuel and the engine would stop running.

Engine lubrication

Another factor in an aerobatic aircraft engine is that of lubrication. It is essential to provide lubricating oil to many points in the engine regardless of the aircraft attitude. In the AEIO 540 engine, inverted oil system hardware is used to adapt oil pickup lines at the top and bottom of the wet sump engine. The system is made up of two major components - an oil valve and an oil separator. Other pieces of hardware are incorporated to ensure that oil is available to the oil pump in either the upright or inverted position. These include a standpipe in the sump that acts as the engine breather during inverted flight, a special adapter or plug at the oil sump suction screen, and other hoses and fittings.

One should keep in mind that these modifications ensure an adequate supply of lubricating oil during normal upright and inverted flight. However, because the oil pickups are incorporated at the top and bottom of the engine, knife-edge flight or flight at very high up or down pitch angles do have some limitations. However, these limitations do not keep them from performing the necessary maneuvers for international aerobatic competition.

Maintenance issues

Engine mounts experience plenty of stress and should be inspected regularly

So what maintenance practices are necessary to keep these special aircraft in the air? Anson Mount, Chief of Maintenance for Klein Tools corporate aviation department, shared some insight into this. One of the aircraft his department maintains is an Extra 300L identical to that of Mike Mancuso. I visited him while they were performing an annual inspection on the aircraft.

Mount started off by saying that performing an inspection on the Extra 300L would be similar to performing an inspection on any other aircraft. You have all of your technical references and all the required inspection items. But there are factors that are more critical in an aerobatic aircraft than a normal one.

"Debris is a critical issue," Mount shared. "When working on any aircraft, it is a good practice to be critical of potential FOD. But in an aircraft such as this, it becomes even more critical. With the excessive maneuvers that this aircraft encounters, something left in it could migrate to the oddest of places. It is crucial to always be aware of FOD."

He shared that seemingly simple tasks take a little more time because of the extra precautions to eliminate FOD. "Drilling something, for example, takes some more time than normal because I like to have a vacuum hose next to the drill bit while drilling to keep stray shavings from falling into the aircraft."

Checking for security

Another thing that Mount pointed out is checking for security of components and wiring. "The aircraft is undergoing some extreme maneuvers and is put under extreme G loads. You would be amazed at how wiring will end up chafing something that is several inches away during extreme maneuvers."

Although security of components and wiring is more critical in an aerobatic aircraft, it is also something to keep in mind when looking at any aircraft. Loose wiring could be subjected to chafing causing a headache or worse if the wiring is chaffed through the insulation causing arc faults.

Engine and propeller

On the engine itself, there are several items to pay particular attention to. Ty Englehardt, a mechanic for Klein shared, "Today's aerobatic competition aircraft are built with only one thing in mind - winning aerobatic competitions. In an unlimited aerobatic sequence, an aircraft may go from 10 positive G's to 10 negative G's in a little over a second. The speed will vary from 250 knots forward to 50 knots backwards. In maneuvers such as flat spins and snap rolls, the aircraft can reach a pitch and/or yaw change of over 360 degrees a second. This puts a huge load on the pilot, airframe, engine, and particularly the propeller."

Flight control rod ends

The Extra 300L has an MT constant speed three-blade propeller. Constant speed propellers require a hollow crankshaft for an oil passageway to provide for pitch control on the prop. Needless to say, these hollow crankshafts are not as strong as their solid counterparts, so attention should be paid to this area.

Englehardt shared, "In certain aerobatic maneuvers, such as a snap roll and power on (flat) spins, the gyroscopic moment of the propeller may exceed 300 ft./lbs. This imposes over 2,000 ft./lbs. of force on a 2-inch prop bolt radius. This force bends the crankshaft back and forth at the RPM that the engine is turning. This combination of back and forth motion is exceedingly stressful to the hollow crankshaft and may lead to failure."

In relation to propeller TBO, Englehardt shared that they rarely reach TBO. "Many manufacturers such as Schnitic and Hartzell recommend that propellers be overhauled twice as often if the propeller is used for aerobatics."

Given the amount of stress that these propellers encounter, it is easy to see why they would recommend the more frequent TBO intervals.

Englehardt also pointed out that the engine mounts take a lot of abuse. "You should also pay close attention to the engine mounts. They tend to wear out quicker than those on a normal aircraft."

Oil system

The air filter on the Extra 300L engine amounts to little more than a screen. This can allow particles to enter the oil system. It's not like a normal aircraft where some particulates may end up settling down in the bottom of the oil sump. These aircraft are flown in almost every possible attitude, and the oil gets tossed around. So oil changes are a regular occurrence on the aircraft at 25-hour intervals. Not only that, but oil analysis is an important part of maintaining these engines.

Mount shared, "Oil analysis can tell you a lot about what is going on with the engine. But it's something that must be done on a regular basis. A one-time analysis is not going to tell you anything. We have our oil sampled on a regular 25-hour interval."

Keeping it clean

Cleanliness is essential on these engines. Keeping the engine extremely clean helps to quickly locate leaks if they occur, helps the engine operate better (more efficient cooling of the fins), and allows for easy inspections. This also keeps FOD from becoming an issue.

Other areas

Englehardt shared that other areas of the aircraft are susceptible to quicker wear on aerobatic aircraft. Flight control attachment points, control rod ends, and bellcranks should be looked at closely during the inspection. These moving parts are subjected to an enormous amount of stresses, and should be looked at closely.

In the end, maintaining a high performance aerobatic aircraft is much like maintaining any other one. There are just a few things to pay more attention to. But some of the practices used in maintaining these aircraft transcend to other types in that they will ensure continued safe operation of the aircraft.

Additional ReSources

Textron Lycoming
652 Oliver St.
Williamsport, PA 17701
(570) 327-7278

Klein Tools, Inc.
7200 McCormick Blvd.
Chicago, IL 60659
(847) 677-9500

MT Propellers