“You can see from the debris whether the engines were producing power at impact—turbines break apart differently when they’re traveling very high,” said Peter Goelz, former managing director of the US National Transportation Safety Board.
And that single fact now informs the meticulous forensic inquiry into the causes of Air India Flight 171’s horrific crash the air disaster that has transfixed the global airline community, not merely for its tragic loss, but for the technological enigmas it poses.
In less than 40 seconds of flight, the Boeing 787 Dreamliner, full to the brim with almost 100 tonnes of kerosene and 242 individuals on board, plummeted into the skies above Ahmedabad, leaving a destruction trail and an engineering wonderland of queries. The probe, being coordinated by India’s Aircraft Accident Investigation Bureau (AAIB) with the involvement of Boeing, GE Aerospace, and global regulators, is focusing on the triad of key areas: evaluation of turbine damage to the engines, high-tech black box data forensics, and the stability of Dreamliner’s flight management control system. The initial physical evidence is revealed in the engines themselves.
Jet turbine engines, which are rotating at tens of thousands of revolutions per minute, deposit characteristic forensic signatures based on what state they are in at the moment of impact. Everybody is in the hot seat who got charred wires and broken blades, and they use metallurgical and fracture analysis methods that are the same ones reported in military turbofan blade failure tests. These techniques include scanning electron microscopy to separate ductile overload, fatigue, and intergranular oxidation fractures all indicative of a different fundamental cause, ranging from high-cycle fatigue to foreign object damage or thermal degradation. For Flight 171, initial indications are that the aircraft’s emergency power system, or the ram air turbine (RAT), deployed before impact.
The RAT, which is a small propeller that detaches from the fuselage to provide backup power, is typically powered by double engine failure or hydraulic pressure loss. Its use jeopardizes the possibility of a combined loss of thrust from both GEnx engines, which, due to its infrequency, brings to mind only a handful of such cases in contemporary aviation. According to the Boeing 787 manual, the RAT can be deployed manually as well, so engine failure cannot automatically be deduced from its deployment, but it is a useful piece of information for investigators establishing the chronology. Electronic analysis is at the second tier.
The Dreamliner’s Enhanced Airborne Flight Recorders (EAFRs) so-called “black boxes” are the backbreaker of today’s crash investigations. These units are being supplied with hundreds, and occasionally thousands, of parameters each second, from engine thrust and fuel flow to the exact position of slats, flaps, and landing gear. Cockpit voice is also captured by the EAFRs, including pilot radio voice communications, background sounds, and even environmental alerts, providing a minute-by-minute chronology of crew activities and system reaction. As Goelz has pointed out, “One of the first accidents I investigated in 1994 had a flight data recorder tracking just four parameters. Today’s recorders capture hundreds if not thousands every second. That alone has transformed the way we investigate crashes” (BBC). This development allows investigators to correlate physical data with computer data.
If the flight data report is that engines were producing full power on impact, then focus will turn to other considerations such as slats and flaps being set to the correct position for takeoff. These surfaces are for producing lift at low speed, and failure to extend them correctly can be the cause of the plane’s failure to climb. Dreamliner’s takeoff configuration alert system is to detect such mistakes, but the EAFR information will validate whether these systems were operating accordingly. If the study finds glitches with the highly automated flight management control system of the plane a package that brings together navigation, performance, and guidance through sensor fusion effects would extend to the worldwide fleet of more than 1,100 Dreamliners.
As Goelz cautioned, “If [the trail leads] to a problem in the flight control system, that would raise some serious issues not only for Boeing, but for the whole aviation sector”. The Dreamliner system includes redundancy and durability, but systematic software or sensor integration flaws might spur regulator action much farther-reaching than Air India. The scope of the investigation goes beyond hardware and software.
Maintenance histories, pilot training records, and operational records are under scrutiny. India’s Directorate General of Civil Aviation (DGCA) has already carried out more intense inspections on Air India’s 787 fleet, discovering no significant safety issues but noting a need for better coordination of maintenance and real-time defect reporting (Aviation Week). This lack of systematic evidence does not eliminate the chance of a sole technical or human mistake on Flight 171, and authorities are examining each replacement piece and maintenance procedure for concealed flaws or chronic maladies. Techniques borrowed from both business and military flight are being used on the wreckage. These include extremely detailed blade fracture analysis, used in turbofan failure investigations to distinguish impact damage from pre-crash fatigue or oxidation damage. They will also test samples of fuel for contamination, scan maintenance records for engine work done close to the accident time, and rebuild the health of the plane over the days and hours leading up to the crash through ACARS transmissions.
The character of this research is equaled only by the character of equipment currently available. New flight data analysis, sophisticated forensic metallurgy, and in-flight defect monitoring have transformed the speed and accuracy of accident reconstruction. As explained by former NTBS administrator Jeff Guzzetti, “They’ll put representatives from the different organizations on these groups, and they will begin to methodically document the wreckage and download the recorders”.
Its result will depend on the convergence of physical science and electronic evidence. The solutions whether directed towards a coincidence of technical malfunction, operator error, or design failure on a system-wide basis will have their foundation in the uncompromising application of engineering principles and the relentless search for every shred of evidence, from the broken tip of a turbine blade to the last line of computer code in the Dreamliner’s flight control system.
