Failure To Feather

Did the pilot know the props on his high-time engines couldn’t be feathered at or below 1000 rpm?


Each aircraft type has its own set of limitations with which operators must comply if it’s to meet its airworthiness and certification standards. The aircraft’s documentation—a collection of manuals, handbooks, placards and revisions—tell us what those limitations are and how we are to fly it. But what if a critical bit of information wasn’t in that documentation? Or what if it was buried somewhere not easily accessible in flight? How would pilots and operators know of it?

One way is through recurrent training, especially the sim-based kind. It’s expensive, but so are accidents. When flying a pressurized piston twin, that training should be obtained from a person who knows the airplane and its limitations, and how it should be flown. That costs money, too.

But no amount of training can cover things not in the official documentation. And procedures we might need in a hurry, like those performed in emergencies, should be readily available. Here’s an example of why.

On June 30, 2012, at about 1620 Eastern time, a Piper PA-31P Pressurized Navajo collided with terrain near Dalton, Ga., following loss of power in one engine. The solo private pilot was fatally injured; the airplane sustained substantial damage from impact forces and post-crash fire. Visual conditions prevailed.

A friend of the pilot assisted him prior to takeoff, and reported the airplane was being flown to a different airport for its annual inspection. The friend did not notice any anomalies with the airplane during the takeoff or the climbout. Video footage showed the airplane climbing out normally prior to the accident.

Subsequently, witnesses heard and saw the airplane flying very low. One witness noticed the right propeller was not turning, while the left engine sounded as if it was running at full power. The airplane pitched up to avoid a power line and rolled to the right, descending below the tree line. A plume of smoke and an explosion followed.

The accident site was about two miles west of the departure airport, in a wooded area. The airplane came to rest upright in a flat attitude on a course of about 102 degrees. All major components were accounted for at the accident site.

Due to damage, the pre-impact position of the fuel tank selectors could not be determined. The landing gear selector and flap selector were each in the up position.

The left propeller assembly’s piston/cylinder had separated from the propeller and was missing. The spinner dome was severely damaged due to frontal impact. The blades exhibited multiple bends, rotational scoring, twisting and leading edge damage consistent with rotation under power at the time of impact. An estimate of power output could not be determined. There were no anomalies noted that would preclude normal operation; all damage was consistent with impact damage.

The right propeller assembly revealed evidence of significant frontal impact. Its blades were bent but did not display rotational scoring. One pre-load plate impact mark indicated the blades were at an approximate 23-degree blade angle, consistent with the start lock blade angle position. There were no anomalies noted that would preclude normal operation; all damage was consistent with impact damage.

The right engine was severely damaged, but all of its cylinders were borescoped; the piston heads did not exhibit damage. All of the cylinders remained attached to the crankcase. The top spark plugs exhibited normal operating signatures. No anomalies were noted during examination of the right engine.

The left engine exhibited similar damage, with all of its cylinders remaining attached to the crankcase. Like the right engine, the left one exhibited normal spark plug signatures The top spark plugs were removed, and their electrodes were intact and exhibited normal operating signatures. Borescoping did not reveal damage.
Both engines were, however, beyond their manufacturer’s recommended TBO, at least in years. The left engine was overhauled on November 10, 1998, and its time since major overhaul as of June 18, 2011, was 580.8 hours. The right engine was overhauled on October 28, 1988, and its time since major overhaul as of June 18, 2011, was 1435 hours. Lycoming recommends 1200 hours or 12 years between overhaul.
The aircraft POH’s (revision date December 4, 1981) engine-failure checklists did not have the feathering procedure, nor the one for engine securing. The engine securing (feathering) procedure advises that the propellers must be feathered before they drop below 1000 rpm. The POH’s latest revision (November 1, 2001) contained no mention of the need to feather the propellers above 1000 rpm.

Probable Cause
The NTSB determined the probable cause(s) of this accident to include: “The pilot’s failure to maintain airplane control following loss of power in the right engine for reasons that could not be determined because of fire and impact damage. Contributing to the accident was the pilot’s delayed feathering of the right propeller following the loss of engine power and the lack of specific emergency procedures in the pilot operating handbook indicating the need to feather the propellers before engine rpm falls below 1000 rpm.”

The airplane hadn’t been flown very much recently, and it’s not clear how proficient the pilot was; probably not very. Whatever happened to the right engine—probably a fuel issue—happened quickly enough that the airplane barely made it two miles. A hot day (39 degrees C) and the windmilling propeller combined to pose performance issues the pilot couldn’t handle. At the end, he tried to avoid wires and entered into a classic VMC rollover.

That the engines were beyond recommended TBO is something of a red herring, one the NTSB didn’t even mention in its probable cause finding. It’s not clear what POH the pilot had. If it was the latest one, it’s conceivable he had no published procedure with which to secure the failed engine and feather the prop. Even if he had the procedure in the older POH, it was buried, Something like that should have been on a quick-reference card, anyway.

Most personal airplanes are relatively easy to fly, and Navajos are not known to have handling vices. But it’s common sense that feathering an inoperative engine needs to be done while it’s still turning at a good clip. It’s likely the correct procedure wasn’t available to the pilot, and that’s a problem.


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