Aircraft Accidents and Lessons Unlearned LXVI: Lauda Air Flight NG004

On May 26, 1991, Lauda Air flight NG004 (LA004), a Boeing 767-300ER, registration number OE-LAV, crashed 94 nautical miles northwest of Bangkok. The airliner was climbing out of Bangkok for Vienna, Austria, on its second and final leg of a flight beginning in Hong Kong, when the aircraft broke up in flight over Suphan Buri Province in Thailand.

The aircraft had experienced an uncommanded left-hand engine thrust reverser (TR) deployment in flight. This event, per the report, produced a loss of lift on the left wing, caused a significant yaw to the left, and reduced the forward movement of the aircraft by disrupting forward thrust, while capturing a large percentage of the left engine’s climb thrust in the thrust reverser cowl. Combined, these events led to high stress loads in the airframe and the inflight breakup of the airliner.

The Aircraft Accident Investigation Committee (AAIC) of the Government of Thailand investigated LA004. The AAIC, “… determines the probable cause of this accident to be uncommanded in-flight deployment of the left engine thrust reverser, which resulted in loss of flight path control. The specific cause of the thrust reverser deployment has not been positively identified.” The AAIC took 787 days to arrive at no conclusion, not even a theory beyond the dead end.

Lauda Air is an Austrian airline; the accident aircraft was registered in Austria where it operated. Why did the LA004 report exclude any mention or reference to the Austrian aviation administration, the Civil Aviation Authority (CAA) who, per the website: ecac-ceac.org/Austria, had been admitted into the European Civil Aviation Conference (ECAC) since 1955. Instead, the LA004 report only referred to the Federal Aviation Administration (FAA) during testing and in citations to regulations. Was Lauda operating under the United States’ Federal Aviation Regulations (FAR)? Did the Austrian CAA or ECAC play any role in the investigation? Where did the LA004 report document it?

LA004 was a textbook example of how unbiased resources are needed to conduct a major accident investigation. The report also highlighted why trusting party members who could stand to implicate themselves in an investigation would not produce impartial conclusions. In 1991, the B767 was becoming a very popular wide-body airliner; extended twin-engine operations – ETOPS – were widening the range twin-engine aircraft could fly from land over oceans. The B767 was a perfect candidate for both passenger and cargo air carriers.

This accident investigation was critical in determining what happened to cause LA004, which crashed over land, its evidence scattered across miles of ground making recovery possible. Five years later, TWA flight 800 crashed eight miles from shore; recovery was arduous in only 130 feet of water. What would an LA004-type investigation be like over an ocean? Even if its location was determined, finding debris would have been impossible. LA004 demanded qualified investigators.

Airframe manufacturer, Boeing took part in the TR malfunction investigation while Pratt and Whitney (P&W), the powerplant manufacturer, hardly participated. Reverser issues raised in the LA004 report referred to P&W alerts, which should have been scrutinized during testing. The LA004 report referred to a P&W Type Certificate Data Sheet (TCDS) note, but otherwise P&W wasn’t involved.

Should P&W have been more critical to the accident investigation? Engine systems could have played a significant part with a bad microswitch or inoperative pneumatic valve. Another ambiguous subject was that while the TRs deployed hydraulically, the report failed to elaborate on if the left-hand TR halves were powered by the left engine (#1 engine) hydraulic pump(s) or were pressurized by another hydraulic system? In this, any hydraulic anomalies could have been eliminated, but the report ignored hydraulics details altogether. Mistakes made during LA004’s investigation were numerous; too many stones were left unturned. The report was disconnected, too many unanswered questions led to more questions.

For example, page 8 had the following quote: “Since August 14, 1990, there were 13 maintenance actions logged on the left engine thrust reverser system, almost always in response to recurring Propulsion Interface Monitor Unit (PIMU) messages of "EEC CH-B REVERSER RNG FAIL" and "EEC CH A/B REV CR-CHK FAIL." Ten of these actions occurred since January 28, 1991.” The LA004 report mentioned – then dismissed – these two messages without explaining what they communicated. The report then continued, “The majority of the corrective actions involved removing and replacing valves or actuators, and adjustments to the system. Typically then the PIMU message would not reoccur for several flights. The most recent known action prior to the accident was on May 25, 1991 at Vienna. At this time, a left engine thrust reverser locking actuator was replaced.” The report said a recurring problem with the TR that (might have?) led to the accident was troubleshot for nine months. Did anyone determine why it was still faulting? Were the alerts limited to the left TR? Parts were replaced to correct the problem, but the problem kept returning. This demonstrated the maintenance habit of ‘throwing parts at a problem’ was ineffective as a technique.

NOTE: It should be obvious that no one with a maintenance background, no one experienced in the process of troubleshooting aircraft line problems, was ever consulted during the investigation.

Assuming no one in Lauda Air Maintenance Control (page 8) or Boeing could track a nine-month-old recurring problem, the next statement made little sense: “Lauda had accomplished all the troubleshooting steps from the Boeing Fault Isolation Manual (FIM) without correcting the problems of the recurring PIMU messages.” Was the Boeing FIM a revised manual? Did the mechanics’ troubleshooting per the FIM, before returning the accident aircraft to airworthiness, reference the FIM or the maintenance manual in the signoffs? If the FIM was a legitimate manual, the LA004 report should have said so.

The questions remained: What were the two alerts that populated the aircraft’s PIMU? What was an EEC CH-B REVERSER RNG FAIL alert? What was an EEC CH A/B REV CR-CHK FAIL alert? LA004 never answered those questions. However, a page 17 statement described events in the cockpit moments before the TR deployment, “The flight appeared normal until five minutes and forty-five seconds after takeoff … At this time the crew began to discuss an event in the cockpit that was later identified as illumination of a REV ISLN indication. The pilot-in-command stated, ‘that keeps coming on.’” The captain was worried about this alert illuminating on the Engine Indicating and Crew Alerting System (EICAS) nine minutes before TR deployment, yet the alert was shrugged off by the first officer. Why?

“The REV ISLN indication could consist of either a REV ISLN amber (yellow) light illumination on the center pedestal or a L REV ISLN VAL advisory amber (yellow) EICAS message or both indications. This indication appears when a fault has been detected in the thrust reverser system.” What kind of fault? Why were they getting an inflight TR fault at all? Why was it dismissed? “It indicates a disagreement between the respective hydraulic isolation valve (HIV) and the associated thrust reverse lever position or an anomaly in the air/ground system.” A TR lever disagree gave an alert … inflight … and no one cared? Did the alert say that both TR halves were not stowed and locked? “No corrective actions were necessary and none were identified as taken by the crew.” Did the crew decide no corrective actions were necessary or was that written in their flight manuals? Did the flight crew ignore an important issue?

Per past maintenance training, standard alerts were color-coded as such: Blue – condition or status, like an entry door open; White – message or note; Amber – caution alert that deserves attention; Boxed Amber – Master caution, more serious attention; and Red – Master Warning, such as a fire warning. Amber alerts were warnings – not messages, as LA004 suggested. Amber warnings demanded the crew’s attention, even if just to cancel the alert. The crew dismissed the REV ISLN for nine minutes, even spoke about how, “… additional system failures may cause in-flight [TR] deployment …” This may have been their critical error, not taking the alert seriously or dismissing it. Was complacency a root cause? The LA004 report never explained how serious these alerts were; the report never cited what type of alerts EEC CH-B REVERSER RNG FAIL and EEC CH A/B REV CR-CHK FAIL were. What authorized maintenance to keep dispatching the aircraft for nine months with alerts returning consistently?

Dismissed or overlooked alerts were important. Why did both maintenance and operations ignore them? In the same paragraph, page 8, “The Boeing Dispatch Deviation Guide cites the Pratt and Whitney Type Certificate Data Sheet E24NE, which permits dispatch for up to 500 operating hours with an EEC maintenance message annunciated.” What electronic engine control (EEC) message? What did it say? If the TCDS was used for the thrust reverser alerts, wouldn’t Boeing’s TCDS be referenced? Why did they use a TCDS? Did this dispatch permission address the B767-300 series flown by Lauda Air?

Per NOTE 19 of P&W TCDS #E24NE, “The PW4000 series engines have been approved to operate with certain faults present in the control system based on satisfaction of FAR 33 requirements and appropriate FAR 25 control system reliability requirements. The following criteria exist as dispatch and maintenance requirements for the engine control system. These criteria are specified in Pratt & Whitney Report PWA-6139 and PWA-6139 Addendum, which defines the various configurations and maximum operating intervals as follows:” The fault levels described showed Level A, no dispatch allowed; Level B maximum dispatch of 20 days; and Level C maximum dispatch of 1000 operating hours. Did Lauda dispatch per the FAA FARs and not CAA certification regulations? Were Austrian CAA rules followed?

TCDS E24NE stated the EEC fault data, its Levels A, B, and C may be different from the aircraft’s interpretation. “The airframe manufacturers may use different nomenclature in adapting these fault categories to the aircraft maintenance and display systems.” The report said the engine and airframe manufacturers used different language to describe the same fault. Was this interpretive information correctly translated to the Boeing Dispatch Deviation Guide (BDDG) to extend operating intervals? Was the BDDG a revised manual? The LA004 report should have described the FIM and BDDG and report if these manuals were acceptable instructions for continuing airworthiness.

Could Lauda have used a Minimum Equipment List (MEL) instead of a TCDS for dispatching this deferrable item? Was the accident aircraft, registration QE-LAV, even using an MEL? Did Lauda’s maintenance control track the TCDS dispatch extensions? How did they track them? Why did the LA004 report only recognize the FAA?

If Lauda Air was following the FAA’s lead, why the BDDG? There was no evidence the BDDG was revised, or its technical data was up to date. What dispatches were extended to 500 hours? There still was no information about all other alerts mentioned. So many unanswered questions.

Which brings the report back to the total dependence on Boeing’s participation in the accident report. The AAIC’s fundamental lack of technical knowledge about the Boeing 767 aircraft and the P&W PW4060 engine, left the investigatory group fully dependent on the forthrightness of the manufacturer. This made LA004’s report suspect, the findings suspect, the probable cause suspect, any information suspect. What was uncovered at all?

The report referenced the Airworthiness Directive (AD) list for applicable follow-up to LA004. AD 91-15-09, AD 91-17-51, AD 91-22-09, and AD 91-22-02, all could not be found in the FAA’s Dynamic Regulatory System (DRS). The latest version of the FAA’s tracking system did not display either AD under their AD numbers, Boeing, or P&W. It is unclear why the Airworthiness Directives did not show up in the FAA’s DRS, the system remains a collection of all FAA Airworthiness Directives, FAA Orders, Legal Interpretations, and Guidance.

LA004 was closed out without any safety improvements. Investigators were out of their element; they took anything the manufacturers told them as fact, without question. They came to a crossroad: Deliver a report that heightened safety or push a report that answered nothing. LA004’s report answered nothing.