How Did MH370’s Flaperon Come Off?

In my last post, I reviewed Malaysia’s analysis of the MH370 debris its investigators have gathered. Not included in that study was the flaperon found on Réunion, as it is being held by the French. So today I’d like to look at what the damage patterns seen on the flaperon suggest about the crash, based on the work done by IG member Tom Kenyon and by a reader of this blog, @HB.

On February 3 of this year, Kenyon released an updated version of his report “MH370 Flaperon Failure Analysis” in which he gives an overview of the flaperon’s structure and how it was damaged. He notes that of the six main structural attachment points of the aircraft, the two biggest and most significant are the flaperon hinges (pictured above). They snapped in the middle:

The lesser attachment points failed in a similar way. That is to say, they did not rip away the flaperon structure to which they were attached.

Kenyon observes:

The location of the failure points of Flaperon hinges is consistent with a large singular lateral force or repetitive lateral (or torsional) movement of the hinges in the inboard/outboard direction. If Flaperon was separated from the Flaperon hinge with forces in forward/aft direction or by applying forces to the Flaperon in the extreme rotated up/down direction (beyond structural stops) then deformation of the Flaperon structure due to such forces would be evident. Significant and permanent deformation of the Flaperon structure does not appear to be present in photographs of the Flaperon.

Recall that two scenarios have been proposed for the flaperon coming off 9M-MRO: either the plane hit the water, or it came off as the result of flutter in a high-speed dive. Neither event could reasonably be expect to produce a primarily lateral (that is side-to-side) force on the flaperon of the kind Kenyon describes.

To raise the level of perplexity, Kenyon points out in other crashes involving 777s, failures didn’t occur at the hinges; rather, the hinges remained intact and the material to which they were attached broke. That is to say, hinges are stronger than the flaperon proper. Here’s an example from MH17:

Kenyon concludes that:

No significant evidence of secondary structural damage excludes a massive trailing edge strike and leads the author to conclude that the Flaperon separated from MH370 while in the air and did not separate from the wing due to striking water or land.

In other words, since the damage isn’t consistent with a crash into the sea, we can deduce that flaperon must have come off in the air. The only conceivable cause would be high-speed flutter. However, on closer inspection the evidence seems to rule out flutter, as well.

The reader who goes by the handle @HB is an expert in quantitative risk assessment in the transportation industry and has extensive experience with composite materials. In a comment to the last post, he observed:

For the flaperon… the lift/drag load is normally passed on the honeycomb panel over the exposed surface area then the primary stucture of the component (the aluminum frame of the flaperon) then the hinges then the primary aluminum stucture of the wing.

In a nut shell, those panels are not designed to sustain any in-plane loads either compressive or tensile. They are just designed to resist bending due to uniform lift load on the surface (top FRP layer in tension and bottom FRP layer in compression, the honeycomb is basically maintaining the distance between the layers without much strength).Those panels can arguably take a little bit of shear load due to drag forces on the top skin (top and bottom forces in opposite direction) but not much due to limits in the honeycomb strength.

The second thing to consider are the GRP properties. The GRP is very tough in tensile mode, much stronger than Steel. In compression mode, it buckles easily and only the honeycomb is preventing this. This is by far the weakest failure mode. If it fails, you will see fibres pulled out link strings on a rope failing under tension. For the skin under compression, you will see sign of compression on the honeycomb but the fibres will have to be pulled out as well. Also, a perfect manufacturing does not exist, there are always delamination (small bonding defects) between the honeycomb and the GRP skin to weaken further the compressive strength.

The hinges are usually much stronger as all the load is passing through them (analogy door hinges).

So you could imagine, if there is a large impact the hinges are expected to fail last. The part of the skin that will buckle is expected to fail first.

@HB here is agreeing with Kenyon: it is baffling that the flaperon came off the plane due to failure within the hinges. But @HB goes further, arguing that this type of damage is inconsistent with flutter:

I would tend to agree that the hinges have been subject to cyclic fatigue… the hinges appear to have been subject to cyclic lateral forces which are not expected in any accidental circumstances (take door hinges, for instance, and imagine the hinge fail after 50 times someone is trying to burst through – you can try at home but it is very unlikely to happen). This of course requires a closer look by experts to double confirm. I cannot think myself of any possible lateral force on this part in the first place but a lateral force that will fail the hinges and not the skin which is weaker is very hard to explain. Try with a wooden door and tell me if you manage.

In a followup comment, @HB observes that even under very strong oscillation the flaperon should not be expected to disintegrate. “If hydraulic power is on, fluttering is unlikely to cause any disintegration. If off, fluttering forces are up and down and the hinges are free to move. Lateral forces, I think, would be small in comparison with the vertical forces and not be strong enough to cause fatigue on the hinges.”

Kenyon concludes his report with a list of six questions and issues generated from his analysis. While all are worthwhile, one stands out to me as particularly urgent:

• Why are the official investigators silent on releasing preliminary reports on their Flaperon analysis? Why would France’s Direction générale de l’armement / Techniques aéronautiques (DGA) release photographic data to ATSB and yet chose not to make Flaperon Analysis findings public after such a long period of elapsed time?

To sum up, a close examination of the flaperon’s breakage points does not yield any comprehensible explanation for how it came off the plane, commensurate with a terminal plunge into the southern Indian Ocean.

This is baffling but unsurprising. Every time we look at the debris data carefully, we find that it contradicts expectations. The barnacle distribution doesn’t match the flotation tests. The barnacle paleothermetry doesn’t match the drift modelling. The failure analysis doesn’t match the BFO data. And on and on.

Something is seriously amiss.