Torn door, loose bolts: how such incidents are possible on planes ? Answers from an expert

Investigations are being carried out on the Alaska Airlines Boeing 737 – 9 MAX, whose door was torn off on January 5. MAXPPP – NTSB HANDOUT

Bernard Prigent, legal expert in damage analysis and aeronautics specialist, answers questions raised by the incidents on the 737 MAX, while the American civil aviation regulator opens a investigation into Boeing.

How can an airplane door tear off like this in mid-flight?

The part that came loose is a panel, a cork door which took the place of what could have been a door.

From the moment this "false door" takes up some play, it offers much greater resistance to air friction. On takeoff a plane travels at approximately 250 km/h, in flight at 850 km/h, we are then 60 times more subject to friction forces. So if there is an imperfect seal, if the false door comes off a little, the friction forces running along the fuselage will rush in and tear it off . 

However, this plane had experienced untimely alarms being triggered during previous flights. They could be caused by failing sensors. Or it could be the warning sign of a depressurization, caused perhaps by this panel which' 39;s more waterproof. They hadn't found the source of the problem.

These alerts had not been taken into account ?

Yes, this plane provided connections over the ocean. Following the alarms, they arranged for it to be assigned to lines flying mainly over land surfaces, so that it could land more quickly in the event of a problem .

Since then, poorly tightened bolts or poorly secured parts have been reported on Boeing 737 MAX 9s by United and Alaska Airlines…

Yes, this would tend to support the fact that there was an assembly problem on these stopper doors. This panel, it&# It is like a patch that is applied to an opening in the fuselage, fixed with four bolts, two at the top and two at the bottom.

If there is one, either broken due to a manufacturing defect, or loosely tightened due to a manufacturing defect assembly, the door takes some play in flight, and the seal is less good.

Bernard Prigent. DR

How is this possible ? However, there are control procedures.

Yes, when the panel is assembled to the fuselage, we put in place these famous bolts, the anti-unscrewing systems,  and we must ensure that the operation has been carried out and verified.

This is valid at all stages of the life of this part, both with the subcontractor who manufactured the system (Spirit AeroSystems),   but also at Boeing which has the opportunity upon receipt of the aircraft section to check that the bolts and their anti-unscrewing systems are in place.

Boeing can then remove the door plug to facilitate access to the aircraft, the passage of people, cables and carpet. He must then ensure, when he puts it back, that the bolts are tight. Quality controls are carried out at all stages of manufacturing. Everyone must be able to demonstrate that they have done them well. 

This incident did not cause any casualties, what would have happened if the cap door had torn off at a higher altitude ?

The depressurization would have been much stronger. At that altitude, we were at approximately 5,000 meters, there is a 25% pressure difference between the inside and the outside.

If we had been at 9,000 or 10,000 meters, that is to say the cruising altitude, we would have had a pressure delta of 75% and there the people standing in the cabin were at 9,000 or 10,000 meters. #39;plane could have been sucked out of the plane, it could have been much more catastrophic.

The advantage was that the plane was going uphill and presumably everyone was strapped in. The seats, with their anchoring to the ground, are designed to withstand such events.

Do these flaws not raise questions in a broader way about the security standards applied in air transport ? This incident occurs on a 737 Max, model already at the center of several controversies after crashes and malfunctions.

They are completely different things. When the Airbus A320 came out, there were concerns after the Mont Saint-Odile disaster. We cannot make generalizations from a single case.

The boss of Ryanair, Michael O’Leary, believes that "Boeing and Airbus must considerably improve their quality control".

It's the very principle of quality, it's doing what we call continuous improvement, it's that is to say that we rely on unfortunate feedback like this to advance inspections and controls that we imagined to be hyper-reliable.

There, if we are dealing with an assembly problem, we see that the controls were not sufficient. We can always do better and that is what Boeing and Airbus will do with this experience. That’s why they don’t shoot at each other.

Is air transport, despite everything, still the safest?

Yes, I have no doubt about that.

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