B741, en-route, Gunma Japan 1985

B741, en-route, Gunma Japan 1985


On August 12, 1985 a Boeing 747 SR-100 operated by Japan Air Lines experienced a loss of control attributed to loss of the vertical stabiliser. After the declaration of the emergency, the aircraft continued its flight for 30 minutes and subsequently impacted terrain in a mountainous area in Gunma Prefecture, Japan.

Event Details
Event Type
Flight Conditions
Flight Details
Type of Flight
Public Transport (Passenger)
Flight Origin
Intended Destination
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
mt. Mikuni, Gunma Prefecture, Japan
Flight Crew Training, Inadequate Airworthiness Procedures
Post Crash Fire
Maintenance Visual Inspection
Airframe Structural Failure, Significant Systems or Systems Control Failure
“Emergency” declaration
Airframe, Air Conditioning and Pressurisation, Flight Controls, Hydraulic Power
Maintenance Error (valid guidance available)
Damage or injury
Aircraft damage
Hull loss
Non-aircraft damage
Non-occupant Casualties
Occupant Injuries
Few occupants
Occupant Fatalities
Most or all occupants
Off Airport Landing
Causal Factor Group(s)
Aircraft Technical
Safety Recommendation(s)
Aircraft Airworthiness
Investigation Type


On August 12, 1985 a Boeing 747 SR-100 operated by Japan Air Lines experienced a loss of control attributed to loss of the vertical stabiliser. After the declaration of the emergency, the aircraft continued its flight for 30 minutes and subsequently impacted terrain in a mountainous area in Gunma Prefecture, Japan.


This is an extract from the official accident Report prepared by the Aircraft Accident Investigation Commission (AAIC) - Japan (presently Aircraft and Railway Accidents Investigation Commission ARAIC):

The aircraft with the captain seated at the right-hand seat and the co-pilot on the left for the purpose of training the co-pilot for position as a captain […] at 1824:35 just before reaching 24,000 feet […] was brought into an abnormal situation which greatly affected continuation of the flight. At the same time, a loud noise like “boom” was heard, immediately followed by at an utterance of “squawk” 77 (meaning emergency code number 7700 of ATC transponder) by both the captain and the copilot. Then, at 1825:21 the captain requested Tokyo Control clearance to descend and maintain 22,000 ft, and to return to Haneda (Tokyo International Airport) on account of occurrence of such an abnormal situation. Tokyo Control inquired which was desired, right or left turn for change in heading for Haneda, and received the response from the pilot that he intended to make a right turn. Tokyo Control accordingly, issued instructions to fly on magnetic course of 90° after making a right turn for radar vector to Oshima […].

Thereafter the aircraft deviated from the course somewhat to the right […] At about this time, unusual phugoid[1] and dutch roll[2] motions began, and these phenomena accompanied by large or small motions continued until just before the crash. At 1827:02 Tokyo Control confirmed the declaration of an emergency and then asked “What is the nature of the emergency?”, but received no response from the aircraft. At 1838:31 Tokyo Control instructed again the aircraft to “take a magnetic course of 90° […]”, but the response “now uncontrollable” was received from the aircraft at 1828:35.

The ATC has given the crew the option to divert to an alternate aerdorome (72 nm from their present position) but the crew has chosen to return to the departure airport - Haneda. In response to the inquiry from the controller is the aircraft able to descend, the crew replied “descending now”. The further communication was in Japanese as suggested by ATC and acknowledged by the crew.

[…] At approximately 1841 hours the aircraft started to descend from altitude of 21,000 feet […] to an altitude of 17,000 feet changing the heading about 360° to the right in about 3 minutes. Thereafter the aircraft continued flight descending rapidly eastward, transmitting “aircraft uncontrollable” at 1845:46 hours, then turned left towards north-east. At 1847:07 the aircraft requested radar vector to Haneda, to which Tokyo Control instructed the aircraft to “maintain heading of 90°. Haneda’s active runway 22”, which was acknowledged by the aircraft. Then, in response to an inquiry “is the aircraft controllable” made by Tokyo Control at 1847:17, “uncontrollable” was answered. At approximately 1848 hours the aircraft turned left at an altitude of about 7,000 feet and flew WNW gradually climbing, and after reaching 13,000 feet at about 1853 hours it started again a descend, and again transmitted “uncontrollable” at 1853:31. At about 1854:19, the aircraft switched communications to Tokyo Approach Control […] at an altitude of 11,000 feet by an instruction of Tokyo Control. At 1854:25 the aircraft requested its position, to which Tokyo Approach gave “55 miles NW of Haneda and 25 nautical miles west of Kumagaya”, which was acknowledged by the aircraft […]. Then, at 1855:05 Tokyo Approach transmitted “Both Haneda and Yokota” are available, to which acknowledgement was made by the aircraft. After this, there was no response from the aircraft to calls of Tokyo Approach as well as Yokota Approach Control."

At 1856:30 hours according to eyewitnesses and Digital Flight Data Recorder (DFDR) data readout, the aircraft struck terrain alongside of a mountain.

Regarding the extensive investigation efforts the Report states:

"Reconstruction was conducted on the airframe wreckage aft of the vicinity of BS [body station] 2200 in the investigation of wreckage of the aircraft, on the assumption that an irregularity occurred in the aft airframe during the flight, from the fact that the vertical fin and a portion of the aft fuselage were discovered from Sagami Bay as well as from the records of DFDR and CVR".

See Figure 1, Lost portions of the vertical fin (stabiliser) - indicated by hatched lines.

Figure 1: Lost portions of the vertical fin. Image Source: Official AAIC Report on the accident.

According to the official Report, the aircraft was involved in a tailstrike incident at Osaka International Airport on 2 June 1978, which damaged the aircraft's rear pressure bulkhead. The repair of the pressure bulkhead did not conform to Boeing's approved repair methods. Boeing procedure calls for one continuous doubler plate (a patch) with three rows of rivets to reinforce the damaged bulkhead, but the technicians performing the repairs on the aircraft used two separate doubler plates, one with two rows of rivets and one with only one row. This reduced the part's resistance to metal fatigue by 70%. When the bulkhead gave way, the resulting explosive decompression ruptured the lines of all four hydraulic systems. With the aircraft's control surfaces disabled, the aircraft became uncontrollable.

In Section (c) Longitudinal Control, the Report states:

"From an early stage after the occurrence of the abnormal situation, it is considered that the elevator had been in floating conditions due to loss of hydraulic pressure […] Under such conditions normal attitude control should have been impossible.

As alternative longitudinal control elements other than the elevator, there are controls either using engine thrust, or flap operation by alternate extension. Although the period of phugoid is long, the driving speed of flaps by the alternate system is extremely slow and therefore it does not contribute to practically suppress the phugoid mode.

It is theoretically possible to stabilise the phugoid mode is brought into existence due to loss of the longitudinal attitude control by operating the thrust lever. It does never mean that the task is easy for a pilot to accomplish."

As to Cause on the accident the Report states:

  • "It is estimated that this accident was caused by deterioration of flying quality and loss of primary flight control functions due to rupture of the aft pressure bulkhead of the aircraft, and the subsequent ruptures of a part of the fuselage tail, vertical fin and hydraulic flight control system.
  • The reason why the aft pressure bulkhead was ruptured in flight is estimated to be that the strength of the bulkhead was reduced due to fatigue cracks propagating at the spliced portion of the bulkhead’s web to the extent that it became unable to endure the cabin pressure in flight at that time.
  • The initiation and propagation of the fatigue cracks are attributable to the improper repairs of the said bulkhead conducted in 1978, and it is estimated that the fatigue cracks having not be found in the later maintenance inspection is contributive to their propagation leading to the rupture of the said bulkhead."

The Report produces numerous recommendations (Referential Matters) beginning at page 130 which record Safety Recommendations in respect of Airworthiness (all made by the US NTSB) and various Safety Actions taken. The full official report is available on the SKYbrary bookshelf (see the link under Further Reading).

Notes of the SKYbrary editor

  1. ^ Phugoid motion (aerospace engineering) - Pertaining to variations in the longitudinal motion or course of the center of mass of an aircraft. See Figure 2.
  2. Figure 2: Phugoid motion. Image Source: Wikipedia

  3. ^ Dutch roll - (aerospace engineering) - A motion of an airplane which consists of simultaneous oscillations of the bank (or roll) angle, the slideslip angle, and the heading angle, and which, when poorly damped, is annoying to passengers and pilots.

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