structural failure in aircraft

By John Thorpe, GASCo's Chief Executive

reproduced from GASCO

Pilots hope, trust and expect that this is an exceedingly rare event because it is generally sudden and almost always catastrophic. In my early days in Flight Test at Filton we had a number of Vickers Valiant V bombers on a modification and upgrade programme. Each aircraft had to be flight tested afterwards to make sure all was in order. Generally two flight observers went along in the three rear facing seats, something I have never got used to as it is alien to humans who have evolved over many generations to face the way they are going, ever since they got on a horse! My last Valiant flight was in the hands of a pilot who finished with a tower flypast and steep climbing turn with four Avons disturbing the peace and the tin-worms holding hands for grim death. Little did we know - a couple of weeks later all Valiants were grounded due to the discovery of one with near failure of the main wing spar. The ones at Filton were summarily chopped up: some had only flown 400 hours.

Valiants being broken up at Filton in 1965

In a recent 20-year period, there were 23 fatal accidents where airframe/structure failure was a factor, 8% of the total. There were a number of causes including maintenance/pre-flight (12 cases), overload VFR (6 cases), death spiral in IMC (4 cases) and turbulence (1 case).

All aircraft have specific limits on the amount of normal operational 'g' that the structure will withstand before it is permanently deformed with a further safety factor on top before failure occurs. Exceed these limits at your peril. They can be exceeded in the very severest turbulence, during badly executed aerobatic or spin manoeuvres particularly if the cg is outside the correct range leading to over-light controls. Another event that easily and frequently leads to airframe failure is loss of control in IMC, either entered deliberately or inadvertently leading to the dreaded death spiral. In the US, which has many more fatal accidents than the UK, certain fast slippery aircraft have a reputation for this including the Beech V tail Bonanza and the Mooney M20.

So how can such catastrophic events be avoided?

Obviously know what the limits are for your particular aircraft and if it is cleared for even limited aerobatics, make sure a 'g' meter is fitted. Get aerobatic training from a qualified instructor, don't be tempted to just 'have a go' like the pilot of a Rallye who managed to wrinkle the wings when he exceeded the normal limit. It needed expensive new wings, much to the Club's displeasure. Read Safety Sense Leaflet No 19 'Aerobatics'.

Wrinkled wings on Rallye 110S7after untrained pilot attempted aerobatics

Stay away from areas where unusually severe turbulence might be expected such as the lea of mountains and cliffs in windy conditions, or the vicinity or interior of a thunderstorm.

A Cuby fuselage minus wings after meeting severe coastal turbulence near the Giant's Causeway, Antrim

If you fly a slippery aircraft and your licence allows you to fly in cloud - slow down, it will give more time to deal with a minor upset and probably keep you at less than Va, the speed at which all control surfaces can be fully used. If you are a VFR pilot, stay out of cloud and if you have an IMC Rating it is to get you out of trouble and is NOT the same as an Instrument Rating.

Fuselage of a PA-28R Cherokee Arrow on the South Downs near Amberley following loss of control in IMC

If the aircraft has to be rigged before flight, check and double check, don't hurry and don't let anyone distract you.

Don't skimp on maintenance, especially potentially difficult inspection of the slowly corroding structure, particularly if the aircraft lives near the coast. The average age of the UK fleet is getting steadily older. Remember that even a minor brush between a wingtip and the hangar door or a straw bale can exert tremendous leverage on the centre section structure. Satisfactory checking of composite aircraft may not be straightforward.

Robin DR400 Regent wing near Almonsbury, Gloucestershire which detached following an earlier collision with a straw bale when landing at Kemble

Don't skimp on maintenance, especially potentially difficult
inspection of the slowly corroding structure