What causes a dive?

It’s now been confirmed that the crash of the China Eastern Boeing 737-800 had no survivors. It’s with great sadness that the news was released. This is a devastating event that will echo down the years.

The Chinese authorities continue to press-on with a sense of urgency. So far, in the tests that have been done, no signs of explosives have been found at the accident site[1]. The rain-soaked hillside terrain continues to make the investigation difficult to conduct. They continue to look for the Flight Data Recorder (FDR) in the mud.

What is known is that the abrupt nosedive into a mountainous area occurred just before the crew would have started a descent to the airport in Guangzhou. If the FDR is recovered and replayed, it should give an indication as to the state of the aircraft as the dramatic upset initiated.

One thing with the Boeing 737 is that it is such a populous aircraft. It has a long history. As such, even given its excellent safety record there’s a large amount of information on previous serious incidents and accidents.

Since, amongst a large population of aircraft, one-off accidents are extremely rare it’s likely that some clues concerning these events are there in the current records. However, knowing where to look is not so easy. Some major events are written-up in detail while others remain only as scant records. A lot depends on the thoroughness of the investigating authorities.

In my previous article, for the purposes of elimination, I listed several common causes of catastrophic civil aircraft accidents. What is it that can cause an aircraft to abruptly nosedive without the opportunity to recover?

For the moment, I’ll put aside the notion that the aircraft crew had that opportunity to recover. There’s such a thing as the startle factor that can reduce a crew’s ability to take emergency actions, but we will not know anything of that factor until the accident recorders are replayed and the recordings well understood.

The two significant factors that are plausible are unrecoverable aircraft control failure and structural failure. It could be that both factors are linked in one way or another. They are certainly both hazardous but somewhat dependent upon degrees and the sequence of events.

A Boeing 737-800 has powered flying controls, as does all similar aircraft types, but it’s not a fly-by-wire aircraft. Therefore, it does not have the flight envelope protection that is available on a fly-by-wire aircraft type.

What can go wrong with those powered flying controls? Putting aside inappropriate crew action, there are both electronic systems and hydromechanical systems that can fail in a catastrophic way. In the event of electronic systems failing, they can be immediately disconnected but that’s not so simple with hydromechanical systems. 

Illustrative of what can go wrong is this serious incident to a Boeing 737-700 in January 2009[2]. This was a post-maintenance check flight and so the crew were prepared. This aircraft’s descent rate was notable, reaching a maximum of 20,000 ft/min.  It all ended well but just imagine the scenario occurring on a routine flight without expectation that anything was wrong. 

[1] http://www.xinhuanet.com/english/20220326/6a8f099fbd7947caa1233da797bf9f1d/c.html

[2] https://assets.publishing.service.gov.uk/media/5422f73640f0b613420005db/Boeing_737-73V__G-EZJK_09-10.pdf

Author: johnwvincent

Our man in Southern England

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