**Lauda Air Flight 004** was a scheduled international passenger flight from Kai Tak, Hong Kong, China to Wien-Schwechat International Airport, Vienna, Austria via Don Mueang International Airport, Bangkok, Thailand. On 26 May 1991, during it's Bangkok to Vienna leg, the Boeing 767-3Z9ER operating the route suffered from a rare uncommanded thrust reverser deployment while flying over the Burma-Thai border and crashed into wooded terrain about ~94 nautical miles northwest of Bangkok, killing all 213 passengers and 10 crew members on board. It is the first and deadliest loss involving the Boeing 767, and as of 2022, Thailand's worst aviation accident.

The positions of the left engine thrust reverser actuators along with data from the electronic engine control (EEC) and the cockpit voice recorder (CVR) indicate that the left engine thrust reverse system deployed while the airplane was at approximately .78 Mach (478 kn, 550 mph, 885 km/h), climbing through 24,700 feet to flight level 310. The preliminary evidence suggests that the reverse event was recognized by the flight crew, but that the airplane departed controlled flight. 

The true cause of the activation of the reverser was never found because all of the wires and the Digital Flight Data Recorder were destroyed in the crash, but it was thought to be caused by a short circuit that opened both valves for a split second, allowing the reverser to activate.

Accident
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Lauda Air Flight 004 took off from Runway 21L in Don Mueang at 23:02 PM ICT (4:02 PM UTC). The aircraft began a normal climb to an initial altitude of 7,000 feet, then was cleared to 11,000 feet by Bangkok Center. The flight followed a departure requiring a left turn upon leaving the airport. Shortly thereafter, Flight 004 was cleared to FL310 (flight level 310, or 31,000 feet). 

5 minutes into the flight, the pilots received a master caution "REV ISLN VAL" advisory on the EICAS display, accompanied with the master caution tone. First Officer Thurner replied with an expletive. Captain Welch replied, "that keeps coming on". The crew discussed the issue as Thurner looked through the quick reference handbook for a solution, but the handbook then quoted "additional system failures may cause in-flight deployment", and to "expect normal reverser operation after landing". There was no further action required. 

First Officer Thurner asked if he should contact the Lauda maintenance crew in Bangkok, and the captain reassured him, saying that it was "just an advisory thing", and that there was "just water, or moisture in there", and moved on; the first officer shortly thereafter tells the captain that the aircraft needs "a little bit of rudder trim to the left". Over the next few minutes, the aircraft continued on its filed flight plan while climbing to FL310. 

At 23:17 PM ICT (4:17 PM UTC), some nine minutes after the master caution advisory, while climbing to FL310 as cleared, the left engine thrust reverser activated. Immediately after, the first officer acknowledged this, stating "Shit, reverser deployed," followed by loud bangs, snaps, and the sound of the overspeed siren. In just 4 seconds, the aircraft banked over 90 degrees to the left and began to descend as it shuddered with the aerodynamic forces. Captain Welch disconnected the autopilot (or it disconnected automatically because of the overbank) and takes manual control, which consisted of continuous sustained elevator nose up inputs. The aerodynamic stresses tore the tail section free of the aircraft, leaving it uncontrollable.

The loss of the tail provided extreme negative loading of the wings and sheared them clean of the aircraft, and the burning remains of Flight 004 crashed into wooded terrain in the Dan Chang District of Thailand at speeds of over Mach 1 and exploded. All 223 passengers and crew died. Shortly after the accident, local villagers began to loot the crash site.

The available recovery time for the crew to rectify the situation was 4 seconds. The crew never could have reacted fast enough to stop the accident.

Passengers and crew
-------------------

| Nationality | Passengers | Crew | Total |
| --- | --- | --- | --- |
|  Argentina | 1 | 0 | 1 |
|  Australia | 1 | 0 | 1 |
|  Austria | 74 | 9 | 83 |
|  Belgium | 1 | 0 | 1 |
|  Bolivia | 1 | 0 | 1 |
|  Brazil | 1 | 0 | 1 |
|  Canada | 2 | 0 | 2 |
|  China | 6 | 0 | 6 |
|  Croatia | 1 | 0 | 1 |
|  Denmark | 1 | 0 | 1 |
|  Dominican Republic | 1 | 0 | 1 |
|  France | 5 | 0 | 5 |
|  Germany | 4 | 0 | 4 |
|  Hong Kong | 52 | 0 | 52 |
|  Hungary | 2 | 0 | 2 |
|  India | 8 | 0 | 8 |
|  Ireland | 3 | 0 | 3 |
|  Israel | 4 | 0 | 4 |
|  Italy | 10 | 0 | 10 |
|  Jamaica | 1 | 0 | 1 |
|  Japan | 1 | 0 | 1 |
|  Mexico | 7 | 0 | 7 |
|  Netherlands | 3 | 0 | 3 |
|  Norway | 1 | 0 | 1 |
|  New Zealand | 1 | 0 | 1 |
|  Philippines | 2 | 0 | 2 |
|  Poland | 1 | 0 | 1 |
|  Portugal | 3 | 0 | 3 |
|  Russia | 1 | 0 | 1 |
|  South Africa | 2 | 0 | 2 |
|  Spain | 1 | 0 | 1 |
|  Sweden | 2 | 0 | 2 |
|   Switzerland | 7 | 0 | 7 |
|  Taiwan | 3 | 0 | 3 |
|  Thailand | 39 | 0 | 39 |
|  Trinidad and Tobago | 1 | 0 | 1 |
|  Turkey | 1 | 0 | 1 |
|  United Kingdom | 2 | 0 | 2 |
|  United States | 2 | 1 | 3 |
|  Yugoslavia | 3 | 0 | 3 |
| **Total** | **213** | **10** | **223** |

**Josef Thurner**, the copilot, once flew as a co-pilot with Niki Lauda on a Lauda Boeing 767 service to Bangkok, a flight that was the subject of a *Reader's Digest* article in January 1990 that depicted the airline positively. Macarthur Job stated that Thurner was the better known of the crew members. Thomas J. Welch, the captain, lived in Vienna, but originated from Seattle, Washington.

Other websites
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* Lauda Air Crash 1991: still too many open questions – Austrian Wings – Austrias Aviation Magazine
* "การสอบสวนอากาศยานประสบอุบัติเหต" Department of Civil Aviation (in Thai) ( Archived 2012-03-11 at the Wayback Machine)
* "Lauda Air B767 accident report" Ministry of Transport & Communications Thailand (in English)
* "Computer-Related Incidents with Commercial Aircraft The Lauda Air B767 Accident". ( Archived 2014-03-27 at the Wayback Machine) University of Bielefeld. 26 May 1991.
* Lauda Air Flight 004 (Index of articles) – *South China Morning Post*
* "flugzeugabsturz\_20jahre.pdf" ( Archived 2011-12-23 at the Wayback Machine) University of Innsbruck. – Includes list of University of Innsbruck professors, assistants, and students who died on Flight 004
* Last flight of the Mozart (Der Todesflug der Mozart, German) – Austrian Wings – Austrias Aviation Magazine
* PlaneCrashInfo.Com – Lauda Air Flight 004
* Cockpit Voice Recorder transcript and accident summary
* Grand Prix History – Biography on Niki Lauda (Contains information about Flight 004)