B738, en-route, near Toyama Japan, 2018

B738, en-route, near Toyama Japan, 2018


On 8 July 2018, a Boeing 737-800 discontinued three consecutive approaches at its intended destination Toyama because, despite unexceptional weather conditions, it was in each case, impossible to achieve or continue a stabilised approach within the operator s applicable criteria. Diversion to the designated alternate was then commenced with just sufficient fuel to reach it without using final reserve fuel. However, en-route the crew became concerned at their fuel status and ATC initially had difficulty receiving their emergency communications resulting in a MAYDAY declaration. An expedited routing then followed with a landing which just avoided the use of final reserve fuel.

Event Details
Event Type
Flight Conditions
Flight Details
Type of Flight
Public Transport (Passenger)
Intended Destination
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
Missed Approach
Approximately 15 nm north of Toyama Airport
Approach not stabilised, Circling Approach, Copilot less than 500 hours on Type, Destination Diversion
MAYDAY declaration, PAN declaration, “Emergency” declaration, Fuel Status, Aircraft/ATC comms difficulties
Damage or injury
Non-aircraft damage
Non-occupant Casualties
Off Airport Landing
Causal Factor Group(s)
Aircraft Operation
Safety Recommendation(s)
None Made
Investigation Type


On 8 July 2018, a Boeing 737-800 (B18667) being operated by China Airlines on a scheduled international passenger flight from Taipei Taoyuan to Toyama, Japan as CI170 was unable to land at destination despite three attempts to do so and diverted to the designated alternate, Chubu. During this diversion, crew concern at fuel endurance in relation to final reserve fuel led to declaration of PAN and then MAYDAY status which after temporary communication difficulties were resolved and the diversion was completed without further event.


The event was classified as a Serious Incident and an Investigation was carried out by the Japan Transport Safety Board (JTSB). The FDR was downloaded and provided information useful to the Investigation and recordings of relevant ATC radar and communications were also available.

It was noted that the 46 year-old Captain, who was PF, had a total of 9,808 flying hours experience which included 4,526 hours on type and that the 34 year-old First Officer had a total of 505 flying hours experience which included 228 hours on type.

It was established that the flight had departed from Taipei with a declared fuel endurance of 4 hours 31 minutes against a planned fuel ‘no delay’ requirement of 2 hours 51 minutes. The ‘Final Reserve’ fuel quantity was pre-calculated as 1031 kg and the diversion fuel to the nominated alternate, Chubu, was 1525 kg. The diversion route to Chubu from Toyama was flight planned as via the Komatsu VOR (see the illustration of the reconstructed flight ground track below) and the weather there was good.

The flight arrived at Toyama via the Komatsu VOR. The weather for the first approach was a wind from 360° at 9 knots, visibility in excess of 10km and cloud FEW (few) at 1000 feet and SCT (scattered) at 9000 feet. This first approach at Toyama was a circling approach to runway 02 and was discontinued at 400 feet agl when the rate of descent exceeded the operator requirements for a stabilised approach. The same approach was then repeated but discontinued 14 minutes later at approximately 100 feet agl when the indicated air speed exceeded the operator requirements for a stabilised approach. A third direct approach was then made to runway 20 but it too was discontinued at 1000 feet agl 13 minutes after the second one because the rate of descent was in excess of the maximum permitted to remain within the stabilised approach criteria. The fuel remaining at the commencement if this go around was about 2700 kg, just above the quantity planned for diversion to the designated alternate, Chubu, which was around 120 nm to the south of Toyama plus a final reserve (2560 kg).

The Captain decided to commence a diversion to the alternate and Toyama TWR instructed the flight to call Tokyo ACC, the latter subsequently identifying the flight on radar 12 nm north of Toyama and instructing it to climb to FL150.Three minutes later, having received instructions to fly a south easterly heading which was contrary to the anticipated routing to the Komatsu VOR, the flight declared a ‘PAN’ requesting a direct routing to Chubu because the fuel quantity remaining was “in a tight situation”. This transmission was not clearly received by ATC and not explicitly acknowledged and when repeated on request, the direct routing request was not prefixed by ‘PAN PAN’ and consequentially not treated as an urgent request. Three and a half minutes after the first ‘PAN’ call, the crew made a second one, requesting a direct routing to Chubu due to the low remaining fuel quantity but this transmission was again not properly received by ATC and again therefore not recognised as involving urgency so the original radar heading instruction was confirmed and the flight instructed to climb to FL230 “to obtain receive better quality and full understandable calls” from it.

The flight ground track from arrival at Toyama via the Komatsu VOR to its eventual arrival at Chubu. [Reproduced from the Official Report]

Four minutes after this, after noting from the flight management computer (FMC) that the estimated fuel on arrival at Chubu would be close to Final Reserve, a third “emergency” communication was made to ATC advising that “remaining fuel quantity was getting critical even if the aircraft directly flew to the airport”. This time, the emergency communication from the aircraft was clear and the controller recognised and acknowledged the emergency status of the flight. A formal MAYDAY declaration was then made and the flight was cleared to route directly to Chubu and it landed there just under half an hour later with 1125 kg of fuel remaining, just above the Minimum Reserve of 1030 kg. An annotated vertical profile of the flight from the final stages of its descent to Toyama to its landing at Chubu derived from FDR data is shown below.

The Investigation made a number of observations in relation to the information gathered:

  • The prevailing weather conditions were “not involved” in the situation which arose.
  • The Operators OM was aligned with ICAO guidance on the matter of low fuel and required that declarations relating to low fuel should initially use the call ‘MINIMUM FUEL’ and then if the fuel remaining on landing will (not may) be less than the quantity designated as ‘Final Reserve’ then a ‘MAYDAY’ declaration shall have the word ‘FUEL’ added. It was noted that the quality of the recorded communications to ATC from the aircraft were not of sufficient quality to determine if the term ‘MINIMUM FUEL’ was used and the crew could not remember if they had done so.

An annotated vertical profile based on FDR data. [Reproduced from the Official Report]
  • In respect of the concerns of the crew that the route they were initially given was not the one they expected and that it was taking them on a longer route when fuel remaining was a concern was unfounded as the route via the Matsumo VOR would still have been shorter than the flight planned diversion routing even if the direct routing which had by then passed this VOR had not been given.
  • All ATC communications from the aircraft except those to Tokyo ACC during initial climb out of Toyama had been clearly received. It was therefore concluded that the temporary problems which Tokyo ACC had in receiving the initial diversion calls from the flight with sufficient clarity to appreciate the degree of concern being expressed in relation to fuel endurance had been a consequence of “complex influences of altitude, topography, weather, or the like”. This matter was not further considered.

The 'Probable Cause of the investigated Serious Incident was formally determined as the landing conducted in the situation that the remaining fuel quantity was close to FINAL RESERVE after emergency communications.

Contributory Factor was that consuming a considerable quantity of the reserve fuel when attempting to land at the destination airport multiple times contributed to the limited remaining fuel quantity at landing.

The Final Report was adopted by the JTSB on 10 January 2020 and published on 30 January 2020. No Safety Recommendations were made.

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