Aviation Safety Letter 4-2003

COPA Corner: Why Do a Walk Around?

by Adam Hunt, Canadian Owners and Pilots Association (COPA)

I recently received a question from some low time pilots. They reported several observed instances where experienced pilots have jumped into their plane and flown away without doing pre-flight inspection. They wondered if pre-flights are something worth doing, or if they are just exercises for student pilots?

Of course most of the time the aircraft has sat, untouched, since its last flight. But an oil leak may have developed, or someone may have done some "hangar rash" to the plane. In some cases, a thief may have siphoned out all the fuel, except a few litres. That could be a surprise on take-off! Pilots have taken off with external control locks in place, or with concrete blocks tied to the tail. It is very important to do a complete pre-flight inspection before every flight.

One of the most important times to do a careful pre-flight inspection is when the aircraft has been through maintenance or when it has just been reassembled after being transported. This story shows just how wrong things can go, for lack of a pre-flight inspection. The worst thing is that the same accident has been repeated more than once, always with the same fatal results.

Even though this story involves a particular aircraft type, the Pterodactyl Ascender ultralight aircraft, the lesson learned is universal. These car-top transportable aircraft are often kept at home and then assembled prior to flight at the airport.

Pterodactyl Ascender ultralight aircraft

There were original manufacturer's investigations following up the official investigations of several early 1980s accidents where a leading edge spar failed in flight, with no other aircraft components failing. In all cases the results were fatal. The official investigations listed these accidents as "Undetermined", but the follow-up factory investigations found the answers. In each case the spar failed just outboard of the inboard spar sleeve junction, where the inner set of rigging cables joins the spar. The spar failed upwards and twisted as it failed, giving a very distinctive signature to the failure.

These spar failures all had the same signatures and the same causes-the inboard compression strut had failed to do its job. Each wing has two compression struts. The compression struts are designed to keep the front and rear tubular spars apart and also to take the wing's inter-spar compressional forces. Without the inboard compression strut in place, both spars will move together until one breaks. The rear spar is prevented from moving forward by the hang cage centering cable, so the front spar is the one that fails.

There are several reasons why the compression strut can fail to do its job. The compression strut mounting brackets, the bolts or the compression strut itself could fail. There are no recorded instances of the failure of any of these parts. In all accidents investigated, the parts mentioned above were undamaged. The most likely reason for these accidents is that the compression strut was not secured during assembly of the aircraft.

Pterodactyl Ascender ultralights are designed for quick disassembly and reassembly and the compression strut is provided in two parts, joined by a sliding bolt lock. If the two compression strut parts are not connected during assembly, or the bolt lock is not slid into place, the result will be a spar failure in flight. The requirement to check this item is clearly outlined in the Pterodactyl Builder's Manual. The Pterodactyl wing sail is provided with four zippers for just this pre-flight item.

The key defence against these kinds of spar failures is a good pre-flight inspection. Special care should be taken to inspect these after the aircraft has been re-assembled or has undergone maintenance affecting the compression strut area.

Your aircraft doesn't have to have "quick disconnect" style compression struts to have critical pre-flight inspection items. All aircraft have items related to control locks, tie-downs, fuel, oil and other fluids plus many damage-sensitive, structural and control-related areas that must be inspected before each flight.

Do you really need a pre-flight inspection before you fly any type of aircraft? You bet your life you do!

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