USAir 427, The last time they got a 737 crash wrong and blamed the pilots

[RacinBob]

I would suggest reading the Wiki article on flight 427 crash closely as it is very interesting and worth a standalone article. Also, I hope this summary of mine is accurate. I will summarize but defer to the article for any details incorrectly reported. https://en.wikipedia.org/wiki/USAir_Flight_427

The Boeing 737 flipped over and crashed on final approach to Pittsburgh and on a perfect day. It all happened in a matter of seconds. This is a link. The flight recorder and investigation could find no cause and the official conclusion blamed the crew (sound familiar?).

But then a similar 737 crash in Charlotte occurred in final approach and they realized something might be up with the rudder control system. A new crash led them to realize that potentially there was a hydraulic control problem that resulted in it the control command to the rudder control valve being reversed and left became full right.

This I would describe created deep concern with the pilot community as there was an unexplained fault in the 737 resulting in loss of aircraft.

Then they had a breakthrough in that in a similar approach a 737 had the rudder reverse but the crew managed to land the plane. The aircraft was impounded and the valve removed for analysis. However after 100's of thousands a cycles of bench testing in the lab. There was nothing unique about the valve that they could find. it would not reverse.

Finally, a test engineer had an eureka moment and realized they were failing to duplicate the conditions of the failure. The control valve was in a non-conditioned compartment and exposed to minus 30f conditions and would be idle during a smooth flight. But in landing, when first used it would be hit by 140f. hydraulic fluid and undergo thermal stress. When they simulated those conditions the valve reversed almost the first time as I recall reading.

The Wiki article explains it best. Investigators later discovered that the recovered accident rudder power control unit was much more sensitive to bench-tests than other new such units. The exact mechanism of the failure involved the servo valve, which remains dormant and cold for much of the flight at high altitude, seizing after being injected with hot hydraulic fluid that has been in continuous action throughout the plane. This specific condition occurred in fewer than 1% of the lab tests but explained the rudder malfunction that caused Flight 427 to crash. The jam left no trace of evidence after it occurred and a Boeing engineer later found that a jam under this controlled condition could also lead to the slide moving in the opposite direction than that commanded. In light of this, Boeing felt that the test results were not real-world and not applicable due to the extremes under which the valve was tested.[13][11] Boeing stated that the rudder reversal was more likely psychological, likening it to examples when a human panics and intends to step on the brake during an automotive accident, but accidentally presses on the gas pedal instead while under duress.[14][11] The FAA's official position was that there was not enough evidence for probable cause of rudder system failure.[15]

After the longest accident investigation in NTSB history — lasting more than four and a half years — the NTSB released its final report on March 24, 1999.[1][16] The NTSB concluded that the accident was due to mechanical failure: The National Transportation Safety Board determines that the probable cause of the USAir Flight 427 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide.[1]

Boeing agreed to redesign the rudder control system with a redundant backup and paid for the retrofit of the entire worldwide 737 fleet.[20] As one of the NTSB's main recommendations, airlines were required to add four additional channels of information — pilot rudder pedal commands — into flight data recorders, with which the FAA gave airlines until August 2001 to comply.[21] In 2016, former investigator John Cox stated that time so far has proven the NTSB correct in their findings due to the absence of a rudder reversal incident since Boeing's redesign.[22]

[Just Jeepin']

Needs that planelopnik tag, yo.

Nice writeup. I just learned about this flight yesterday in an HN discussion, which is why I dropped that tidbit into the MAX piece.

[annoying_salman]

I remember watching National Geographic's Air Crash Investigations about this 737 problem, very interesting analysis on that episode.

[annoying_salman]

@just-jeepin said in USAir 427, The last time they got a 737 crash wrong and blamed the pilots:

Needs that planelopnik tag, yo.

Nice writeup. I just learned about this flight yesterday in an HN discussion, which is why I dropped that tidbit into the MAX piece.

Do we use Jalopnik-style tag for suitable category here? Or is there Hyphen-specific naming scheme that replicate Jalopnik one?

[ttyymmnn]

@annoying_salman So far at least, "Planelopnik" is the tag for posts such as this.

[Just Jeepin']

@annoying_salman Oppo typically reuses a few from Jalopnik like Foxtrotalpha but mostly have our own...but nothing is formalized, just ad hoc.

Despite the lack of formalization, I like to browbeat shame people into adding planelopnik and spacelopnik and shiplopnik because why not?

[annoying_salman]

@ttyymmnn @just-jeepin I see. Thanks for the information!

[RacinBob]

The interesting takeaways from this for me are;

1. It is convenient but usually wrong to blame the pilot. Humans are fallible, however millions of successful flights suggest that unless there are severe problems with the person, (Air France off Brazil for example) the pilot is not the primary cause.

2. Unless your tests duplicates the environment and use cycle, they may not give the answers looked for. Anybody involved in testing should first try to understand the use of the thing being tested. In this case, they bench tested the plane crash valves with no regard to operating temperatures. Once they froze the valve and boiled the hydraulic fluid, the failure was easily reproduced. Lab tests without field as built conditions are kind of worthless when failures have catastrophic results.

[ttyymmnn]

@racinbob Air & Space Magazine has an excellent article about the investigation that finally pinpointed a problem with the hydraulic valve that actuated the rudder. It's definitely worth a read. But here is the money quote:

It went like this: It was a smooth flight from Chicago to Pittsburgh, so there was not much movement of the yaw damper, which automatically moves the rudder to compensate for the onset of yaw. That lack of movement might have allowed particles to build up in the hydraulic fluid. There could have been a modest thermal shock to the PCU because of overheated fluid from a hydraulic pump—not enough to set off a warning to the pilots but enough to make the cold valve suddenly expand.

The PCUs on other 737s might have tolerated that without trouble. But the valve on this particular USAir plane was especially tight. The thermal shock and the contaminants caused a jam. And the jam happened when one slide inside the valve was slightly off center and more likely to reverse. The pilot or the yaw damper was commanding the rudder to go right, but it went hard over to the left.

All of this occurred at the most vulnerable speed for a 737: 190 knots. The pilots compounded the problem when they pulled back on the control column, which made the plane lose speed and stall. The plane spiralled down and crashed into a hill.

It always takes a chain of events to cause a crash. In this case, it took wake turbulence from a Delta 727, the startling of the pilots, the fact that the plane was flying at the crossover point, the uniqueness of the valve, the jam and reversal, and the mistake of pulling back on the stick. If any one of those things had been different, the plane would not have crashed.

https://www.airspacemag.com/flight-today/probable-cause-29233123/?all

[RacinBob]

Thankyou for the Air and Space reference. I get Air and Space and that must of been where I read it. Scary that I am recalling something from a story that I read 18 years ago, probably with a glass of wine in my hand when I read it.

Thanks
Everybody interested in this should read https://www.airspacemag.com/flight-today/probable-cause-29233123/?all

[ForSweden]

Fun fact; this was (I think) the first investigation where the NTSB investigators used biohazard personal protective equipment...poorly.

[Rusty Vandura]

@racinbob Good morning Bob, here is a link to an excellent write-up about the rudder valve malfunction that appeared in Smithsonian's Air & Space Magazine in 2002. Clearances in that part in the ten-thousandths of an inch.

[Rusty Vandura]

@ttyymmnn See my post below... Great minds think alike.

[StuckMTB]

Fascinating reads! There goes my afternoon.