From: EPI Inc.       Contact: tech@epi-eng.com

 

  Race Engine Technology Magazine CURRENT ISSUE of Race Engine Technology Magazine

Race Engine Technology Magazine

CURENT ISSUE Contents BACK ISSUE Summaries SUBSCRIBE

 

Valid XHTML 1.0 Transitional Valid CSS!  
 

Propeller Damage

The very dangerous consequences of even minor prop damage

It is an unfortunate fact of life in aviation that accidents occur. There are many forms of aircraft accidents which can inflict potentially serious damage to the propeller and, by extension, to all the internal components of the engine system.

The term "engine system" includes all the rotating and reciprocating parts of the engine proper, AND the PSRU (if applicable), AND all driven accessories (prop governor, distributors, magnetos, pumps, etc.)

The picture below shows an obvious example of severe propeller damage. However, you should be aware that serious damage to the entire engine system can occur during events far less serious than what is shown in this picture.

V8-powered Lancair-4P after accident

In the world of certificated aircraft, there are required procedures to be followed in the event of a prop strike. These procedures are well-established, and have been developed mainly from the hard school of experience.

In the experimental community, there appears to be a somewhat cavalier attitude about such events, apparently based on denial and wishful thinking. In fact, after the accident pictured above, there was a strong opinion among the people directly involved that no inspection to any engine system parts should be needed because the prop involved wasn't a metal prop!

The discovery and correction of potentially fatal damage to engine systems resulting from a prop strike incident is the subject of a recent EPI Service Bulletin (hyperlinked below).

A portion of this Service Bulletin is presented here in order to give the mose exposure possible to the seriousness with which ANY damage to a propeller should be treated. These definitions and procedures have evolved from the experience gained from post-accident investigations over the course of many, many years. Consider them to have been written in blood.

The following definition of PROPELLER STRIKE is taken from a blending of the requirements stated in Lycoming Service Bulletin SB 533-A, Continental Service Bulletin SB96-11, and AD-2004-10-14.

A propeller strike is defined as follows:

  1. Any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades;
  2. Any incident during engine operation in which the propeller impacts a solid object which causes a drop in RPM and also requires structural repair of the propeller (incidents requiring only paint touch-up are not included). This is not restricted to propeller strikes against the ground, and although the propeller may continue to rotate, damage to the engine may result, possibly progressing to engine failure.
  3. A sudden RPM drop while impacting water, tall grass, or similar non-solid medium, where propeller damage is not normally incurred.
  4. Any propeller strike occurring at taxi speeds and during touch-and-go operations, involving any propeller tip-to-ground contact.
  5. Any situation where an aircraft is stationary and the landing gear collapses causing one or more blades to be bent or substantially damaged, or where a hangar door (or other object) strikes the propeller blade. These cases should be handled as a sudden engine stoppage because of potentially severe side loadings on the propshaft flange, front bearing, and seal.

Whereas the definitions stated above are intended to be unambiguous, the reality of aircraft ownership is that those definitions are likely to form the genesis of endless quibbling about such arcane trivia as:

  1. How sudden is "sudden",
  2. How tall is tall grass?
  3. How deep does the water have to be?
  4. How much of an RPM drop? (50 RPM, 500 RPM) ?
    {Were you really watching the tach when the prop hit?}

These arguments are specious and merely constitute an attempt to rationalize out of doing the required inspections. And, even though a large number of such inspections find no damage, the consequences of undetected damage from a "prop strike" can be grave.

(If you doubt the susceptibility to such damage, take a look sometime at the tiny dowel that locates the accessory drive gear on the end of a Lycoming crankshaft.)

The information contained in the complete EPI Prop Strike SERVICE BULLETIN should be of interest to ALL operators of propeller-driven aircraft.

ONE EXAMPLE

The picture at the top of this page will, undoubtedly, bring some form of gut-wrenching to all builders. It was taken just after an accident which resulted from an unfortunate power loss in a brand-new, V8-powered Lancair 4P, on its way to OSHKOSH-2001. According to many people who saw this aircraft (prior to this event), it had the potential to capture the Grand Champion award.

The builder completed this aircraft in July, 2001. It was subsequently inspected and signed off by the FAA. The initial test flight program was conducted by Lancair Factory test pilot Don Goetz. During the initial flight hours, the builder received transition training from Don, and continued on to fly off the remaining test hours. The FAA then removed the restrictions, and the aircraft was prepped for the flight to Oshkosh.

In preparation for the trip to Oshkosh, the crew performed a final inspection of the aircraft, which included removal of the cowling and a thorough inspection of the engine system (EngineAir turbocharged V8 with EPI Mark-9 PSRU).

Early the next morning, the builder and a friend boarded the aircraft and departed for Oshkosh (on a flight plan). While climbing through 8,000 ft. on the way to the assigned cruising altitude, the engine began to gradually lose power, and ultimately stopped producing any power at all. The only indication of malfunction was an extremely low MAP, suggesting at first, a broken throttle linkage.

The pilot (and builder) handled the situation by the book. First and foremost, while initially analyzing the situation, he FLEW THE PLANE. Maintaining best-glide airspeed, he contacted ATC to declare an emergency, located potential landing sites, selected the best, and flew toward it.

Note that, during this adventure, the prop is windmilling and driving the engine through the PSRU, creating the effect of a large vacuum pump (MAP = 5" HG) and a very effective speed brake. In a normal power-off descent, this aircraft, with roughly 30 PSF wing loading, will manifest a descent rate unfamiliar to pilots accustomed to small GA certificated aircraft. In this situation (with the prop driving the engine at very low MAP), the descent rate necessary to maintain safe airspeed would be daunting to all but the most experienced pilots.

This situation has all the elements for a tragic outcome:

  1. a total loss of power,
  2. a significant increase in drag,
  3. a pilot with very little experience in high-wing-loading aircraft,
  4. the added emotional burden of a pilot who has just invested a significant fraction of his life in the construction of this potentially-prize-winning aircraft (not to mention the MONEY involved).

Fortunately, the weather was good.

To his great credit, this pilot overcame all these problems, maintained control of the aircraft and delayed deployment of flaps or gear until he was assured he could make his targeted runway.

Unfortunately, there was not quite enough energy in the system for him to reach the approach-end of the targeted runway. The pilot realized he would be a bit short, kept the aircraft configuration clean and fought off the strong impulse to try and milk a few more feet of glide. He maintained control through a premature touchdown, a few hundred feet short of the runway. The aircraft came to a controlled stop a few feet after sliding onto the pavement, and both occupants emerged unharmed (picture above).

The cause of the power loss was interesting. During the post-accident engine system was disassembly, the intercooler was found to be completely blocked by a large mass of ground paper particles, having the same color as the paper towels used during the cowl-off inspection. Further searching revealed a fault in the alternate-air system which, apparently, allowed a paper towel (probably left in the engine bay during the inspection) to be ingested by the turbocharger, pulverized by the compressor, and discharged into the intercooler. The sharp edges and small passages in the intercooler quickly collected most of these particles, and ultimately blocked it completely. There was no damage to the engine system.

The EPI Mark-9 PSRU and Woodward prop governor were inspected in accordance with EPI Service Bulletin 260002. After this complete disassembly and thorough inspection, both the PSRU and the governor were found to have suffered no detectable damage. The PSRU bearings and seals were replaced during the rebuild simply as a matter of good practice.

This aircraft was on display at Sun n'Fun 2002, and did daily fly-by demonstrations and demo-flights for people interested in the propulsion system.

<< Return to: Contents   Go to: Top of Page  Next Subject: Direction of Prop Rotation >>