Following the second crash in less than 20 weeks of a Boeing 737 MAX, and the worldwide grounding of the aircraft type, lawmakers in the US have demanded a “rigorous investigation” into how the model was certified by the US Federal Aviation Administration (FAA).
After the loss of Ethiopian Airlines flight ET302 on 10 March, less than 20 weeks after a similar tragedy involving Lion Air in Indonesia in October 2018, attention has focused on a stall-protection system.
Software installed on the aircraft forces the nose of the Boeing 737 MAX down when a sensor detects a potentially dangerous angle between the wing and the airflow. If the sensor is faulty it can cause the nose of the aircraft to tilt down despite the pilots’ efforts to control it.
The software was deployed because of changes to the aircraft balance and aerodynamics with newer, larger engines made a nose-up attitude more likely.
“It improves the behavior of the airplane in a non-normal part of the operating envelope,” says Boeing.
But the change was not considered significant enough to require additional training for pilots switching from earlier versions of the 737.
The chair of the House committee on transportation and infrastructure, Peter DeFazio, and the chair of the subcommittee on aviation, Rick Larsen, issued a joint statement saying: “There must be a rigorous investigation into why the aircraft, which has critical safety systems that did not exist on prior models, was certified without requiring additional pilot training.
“We plan to conduct rigorous oversight with every tool at our disposal to get to the bottom of the FAA’s decision-making process.”
Aviation safety experts talk of the danger of “regulator capture”, in which officials responsible for overseeing safety become too close to the manufacturers and fail to be sufficiently challenging.
In the 1970s, a cargo-door fault was known to present a risk to the McDonnell Douglas DC-10, but no action was taken until one of the worst disasters in aviation history: the loss of a Paris-Heathrow Turkish Airlines flight with 346 deaths.
On Tuesday, when the Civil Aviation Authority banned the Boeing 737 MAX from UK airspace, the manufacturer insisted: “We have full confidence in the safety of the 737 MAX.
“We understand that regulatory agencies and customers have made decisions that they believe are most appropriate for their home markets.”
The FAA said: “Our review shows no systemic performance issues and provides no basis to order grounding the aircraft. Nor have other civil aviation authorities provided data to us that would warrant action.”
The regulator said new evidence indicated similarities between the Ethiopian and Indonesian tragedies that “warrant further investigation of the possibility of a shared cause for the two incidents”.
The FAA’s acting administrator, Daniel Elwell, told reporters: “It became clear to all parties that the track of the Ethiopian flight was very close and behaved very similarly to the Lion Air flight.”
The new evidence was data captured by a private company, Aireon, which tracked the flight with a satellite-based Automatic Dependent Surveillance-Broadcast (ADS-B) system.
It shows that the pilots experienced problems with the aircraft immediately after take-off. The aircraft climbed very slowly yet increasingly fast, which indicates the crew may have been confused by conflicting instrument readings and false stall warnings.
Boeing said: “The 737 MAX is a safe airplane that was designed, built and supported by our skilled employees who approach their work with the utmost integrity.”
The planemaker says that since the Lion Air crash, it as been developing a ”flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer”. It will limit the amount that the flight control system can apply the nose-down manoeuvre”.
“Boeing has been working closely with the Federal Aviation Administration (FAA) on development, planning and certification of the software enhancement, and it will be deployed across the 737 MAX fleet in the coming weeks.”
The leading aviation commentator, Bjorn Fehrm, asked in response: “One now ask why such check and limits were not implemented in the first place?”
“The signals were used to trigger powerful and potentially dangerous functions in the flight control system.”
He said the software “can overpower the pilot’s elevator control and render the aircraft uncontrollable”.
Boeing said: ”We are doing everything we can to understand the cause of the accidents in partnership with the investigators, deploy safety enhancements and help ensure this does not happen again.”
The aircraft is unlikely to be certified to re-enter service until May at the earliest, which could cause significant disruption to travellers.