By Tony Collins
Following the Ethiopian Airlines crash, the US Federal Aviation Authority says the Boeing 737 Max 8 is airworthy. Why then have several countries – including the UK – grounded the planes? Why do they not trust the FAA’s assurances?
The following is part of a Campaign4Change article published in 2017. It shows how it may take many years to establish a link between major incidents, particularly when the integrity of a large organisation’s equipment is in question. It also shows how the seemingly impossible – from a designer’s perspective – can happen.
From Campaign4Change 21 March 2017:
Dozens of families gathered in the ballroom of a Hilton hotel to hear independent investigators announce the most likely cause of an air crash that killed 132 air passengers.
Some wondered whether official investigations into air crashes always ended up protecting powerful corporate interests. For several years the manufacturer Boeing had denied that a technical malfunction was the cause of the crash. It blamed the pilots.
This was the longest inquiry in the history of the National Transportation Safety Board, an investigative organisation funded by the US government. Congress has mandated the Board’s independence and objectivity.
At first, each Boeing 737 incident was treated as a single unique event. In the absence of any clear evidence of a technical malfunction, suspicion fell on the pilots.
The 737 is, after all, the best-selling commercial jet airliner in history. It has an extraordinary safety record.
Then evidence began to mount that various 737 incidents might have been linked.
After thousands of tests over several years, air crash investigators made a discovery – that a particular technical malfunction could, after all, have caused the incidents.
It was an intermittent malfunction – and one that occurred in a rare set of circumstances. It left no trace. It might have caused a succession of seemingly-unique major incidents.
Now the final verdict on the likely cause of USAir Flight 427’s destruction was imminent. As families sat in silence at the Hilton Hotel, Springfield, Virginia, five board members of the National Transportation Safety Board voted – in public – on whether they accepted the findings of their staff investigators who’d pointed to the likely cause being a technical malfunction, not the pilots.
The vote was unanimous; and some relatives wept. The probable cause was not the pilots. It was “most likely” to have been a technical malfunction.
Boeing accepted the final report into the crash of Flight 427. “We respect the Board’s opinion,” said Boeing after the vote. It made rudder-related design changes that eventually cost more than $100m.
Investigations into rare crashes of 737s show that it’s possible for a major corporation to be mistaken when it clears its own equipment and blames the equipment’s human operators.
The 737 investigations found that “no evidence of a technical malfunction” did not necessarily mean “no technical malfunction”.
The UK government reached a similar conclusion at the end of a campaign by families to set aside an RAF finding of gross negligence against two pilots, Flight Lieutenants Jonathan Tapper and Rick Cook, who died when a Chinook helicopter, ZD576, crashed on the Mull of Kintyre in June 1994.
For 16 years the RAF and Ministry of Defence insisted that there was no evidence of a relevant technical malfunction on the last flight of Chinook ZD576. They blamed the pilots for the crash. But leaked MoD technical papers established that the Chinook’s engine computer systems could fail in unpredictable ways – sometimes intermittently – and leave no evidence.
In the end – after a 17-year campaign for justice by the pilots’ families – the UK government set aside the RAF’s finding against Tapper and Cook, mainly because of doubts over whether the pilots or technical malfunction, or a combination of both, caused the crash.
Arguably, the Chinook and 737 controversies established the principle that, despite the absence of firm evidence of a technical malfunction, a major incident could still be caused by one, or a series of them.
In the case of the 737 incidents, the suspect component at the centre of investigations, a power control unit, was based on an old design (certified in the 1960s) – and straightforward in its operation.
Boeing told the National Transportation Safety Board that, following the crash of Flight 427, there was a lack of evidence of technical malfunction. Boeing pointed to evidence of the actions of pilots.
“There is no evidence to support a conclusion that an uncommanded full rudder deflection occurred (the rudder moving in the opposite direction to that commanded by the pilots).
“While there is not conclusive evidence of a crew-commanded, sustained left-rudder input, such a possibility is plausible and must be seriously considered, especially given the lack of evidence of an airplane-induced rudder deflection.”
On crashes of 737s, Boeing said,
“There is no data to indicate that the Eastwind Flight 517 event, the United Flight 585 accident, and USAir Flight 427 accident were caused by a common airplane malfunction.” [Boeing had argued that each incident was different.]
In a separate submission to the National Transportation Safety Board, the manufacturer of the 737’s suspect power control unit, Parker Hannifin, made a point similar to Boeing’s.
“In sum, after years of one of the most critical examinations in aviation history, there is no evidence that the main rudder PCU [power control unit] from Flight 427 malfunctioned or was other than fully operational.”
But the National Transportation Safety Board, as a statutory authority, had the last word.
Its conclusion did not coincide with the view of Boeing or Parker Hannifin.
It said the most likely cause of the crash of Flight 427 was that the rudder moved in the opposite direction to that commanded by the flight crew. The final investigation report said,
“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 [full aerodynamic 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 over-travel of the primary slide.”
Boeing contested the NTSB’s draft finding that blamed technical malfunction for 737 incidents and crashes – but Boeing had to abide by the independent board’s final decision.
The NTSB is independent of manufacturers. It even has the power to exclude equipment owners from participating in the inquiry.
In 2010 American Airlines was excluded from participating in an investigation into an incident involving one of its 757 aircraft because its technicians downloaded and accessed information from the plane’s black box [digital flight data recorder] before it was examined by independent investigators.
US regulations require that the National Transportation Safety Board is the first to see, download or access information from the black boxes.
A Board press release criticised American Airlines. It said,
“Although a thorough examination by our investigators determined that no information from the DFDR [digital flight data recorder] was missing or altered in any way, the breach of protocol by American Airlines personnel violates the Safety Board’s standards of conduct for any organization granted party status in an NTSB investigation.
“Because maintaining and enforcing strict investigative protocols and procedures is vital to the integrity of our investigative processes, we have revoked the party status of American Airlines and excused them from further participation in this incident investigation.”
737s are incredibly reliable
The 737 is in use in 111 countries. Its reliability record is the best in the world. On average more than 2,000 737s are in the air at any one time. It has carried 17 billion passengers – about twice the world’s total population. It has flown about 120 billion miles, the equivalent of 640 round trips from the earth to the sun.
The design of the 737 rudder system had been considered fail-safe. It was thought it would work properly even when problems occurred. The system had built-in “redundancy”. Every lever inside the lower power control unit had a second lever that moved in concert, in case one should break. There were two hydraulic systems in case one should fail. There was a standby actuator in case the main power control unit stopped working.
Even so, after thousands of tests, investigators found it could fail in very rare circumstances.
After the unexplained crash of Flight 585, the National Transportation Safety Board kept tabs on 737 rudder problems even without evidence they were the likely cause of any serious incidents.
The existence of the National Transportation Safety Board is a check against parties protecting their own corporate interests, namely the reputation of their equipment, after a major incident.
Would the conclusions of the investigations into the 737 incidents have been different if Boeing had been the authority in charge of the final report?
A useful book on the crash of Flight 427 is by Bill Adair, which is an inside account of the 737 rudder incidents. He had access to all the main parties involved.
Also useful is the final report of the National Transportation Safety Board into the crash of Flight 427. It contains Boeing’s submission.