Description

On 21 December 2023, a Boeing 737-800 (OY-JZN) being operated by Jettime on a non-scheduled international passenger flight from Hurghada to Billund was making a go-around after encountering turbulence on approach in night VMC when a selected retraction of flaps was followed by the flaps locking in a mid position. During the subsequent diversion to alternate Copenhagen using an increased fuel burn due to deployed flap drag, it became clear that the flight was likely to arrive there with less than minimum reserve fuel so a MAYDAY was declared. The flight was completed without further event.

Investigation

A Serious Incident Investigation was carried out by the Danish Accident Investigation Board (AIB). Relevant data from the CVR was overwritten because it was not isolated following the aircraft’s arrival in Copenhagen. The FDR was not downloaded but the aircraft operator’s download of the QAR was shared with the Investigation team and was used as the source of relevant data along with recorded ATC communications data which were available.

The 51 year-old male Captain had a total of 12,768 hours flying experience which included 3,789 hours on type and the 53 year-old female First Officer had a total of 11,080 hours flying experience which included 5,100 hours on type.

What Happened

Departure from Hurghada with 185 passengers and crew on board was delayed by 28 hours by rectification of a malfunctioning leading edge flap proximity sensor which had occurred during the previous flight. The flight crew updated their OFP with assistance from the aircraft operator’s OCC which gave the minimum fuel required as 16,814 kg. However, based on the latest forecast en-route winds, a forecast storm over Denmark at their new ETA and the previous technical malfunction, fuel was uplifted to take the departure fuel to 18,000 kg. It was noted that they “did not succeed in updating the OFP” with the latest en-route winds they had obtained from their EFBs.  

At the approximate mid-point of the flight, a routine fuel check indicated that the estimated destination fuel remaining had decreased from 4,000 kg to 3,200 kg and it was decided to change the OFP destination alternate from Gothenburg to Copenhagen as it was both closer and was reporting more favourable surface wind conditions.

On arrival in the vicinity of Billund, the aircraft was established on the runway 27 ILS with a recorded 3,278 kg of fuel remaining. Landing clearance was given with a wind check of 310°/36 knots varying between 17 knots and 50 knots and accompanied by clearance to climb to any desired altitude in the event of a missed approach. The aircraft remained stabilised passing both 1,000 feet agl and 500 feet agl but at 300 feet agl, an EGPWS windshear warning was activated and a go-around was commenced. TOGA thrust was selected, the landing gear selected to up and the flap lever moved from 30° to 15°.

As the aircraft climbed through 725 feet agl, a trailing edge flap disagree condition was annunciated with the flap lever position at 15° but the flap position indicator showing them stopped at 13°. The aircraft was levelled off at 4,000 feet and radar vectored onto an easterly heading as the crew ran the ‘Trailing Edge Flap Disagree’ NNC procedure. Having established that they had a stuck flap condition, they advised that they would divert to Copenhagen.

After being cleared to climb to FL 120, the flight was transferred to Copenhagen Control. Further climb to FL180 was obtained and the stuck flap condition and a resulting higher than normal final approach speed were advised along with a request for “some kind of priority for runway 30”. An abbreviated approach track to that runway was confirmed and the crew then made a revised fuel calculation based on the non normal configuration. This indicated that 20 minutes flight time remained with fuel endurance only 38 minutes, meaning that aircraft was likely to land with less than the required minimum 30 minutes of final reserve fuel. A ‘MAYDAY FUEL’ was therefore declared and ATC responded by alerting the crew to the alternative of a landing at Roskilde. However, having obtained the latest Copenhagen weather, it was decided to continue there and the flight was completed without further event. The actual fuel used from go-around at Billund to landing at Copenhagen was found to have been recorded as 2,091 kg, significantly more than the aircraft operator’s calculation for the same flight without the flaps extended of 1,478 kg. The actual remaining fuel endurance on landing was found to have been 27 minutes.

Why It Happened

It was noted that in order to prevent flap structural damage in the event of a flap overspeed condition, the aircraft had a Flap/Slat Electronic Unit (FSEU) which, if sensing a flap overspeed condition would command the flap load relief solenoid valve to retract the flaps by one position. However, the FSEU had a built-in protection which would shut down the flap drive system if flap position differed from a flap selector position which was activated during this auto-retract command. QAR data showed that during flap retraction following the go-around at Billund, there had been four flap overspeed events, “one of which coincided with a flight crew flap handle movement”.

Boeing advised that during some early pre-delivery test flights of the aircraft type they had found that the flap load relief solenoid valve had failed to reset after the FSEU began a flap load relief retraction if the flight crew commanded a similar retraction at about the same time using the flap handle. After an in-service report of the same occurrence, this valve had been redesigned and the new version installed on new production aircraft. At this time (2013), operators of older aircraft were advised by Boeing in a Fleet Team Digest that the older valve design was subject to a high friction issue but could be replaced on an attrition basis so that if operating normally in service, it “did not need to be replaced”.

Jettime replaced the legacy design solenoid valve involved with the currently upgraded one and the aircraft was released to service with Boeing agreeing that failure of the older-type valve was the most likely explanation for the flap drive system shutdown. It was noted that the flight crew had “opted not to go beyond the NNC and attempt to retract the flaps in order to reduce the unexpectedly increased fuel consumption” and that the AIB “considered this decision to be rational”.

The narrative Conclusion of the Investigation was as follows:

• Following a missed approach due to windshear at the destination aerodrome, a flap overspeed occurred during initial climb-out. 
• The FSEU commanded an automatic flap retraction while the flight crew simultaneously moved the flap handle to position 15°. A flap solenoid valve failed to reset, and the FSEU removed power to the flap drive system when the flaps were retracting at position 13°.
• Due to the weather conditions, the flight crew decided to divert to the destination alternate aerodrome. 
• The increased fuel consumption due to the flap 13° non-normal aircraft configuration reduced landing fuel to less than the required minimum final reserve.
• The flight declared a fuel emergency leading to ATC priority and direct routing.
• The aircraft landed with 119 kg less than the required minimum final reserve fuel of 1,118 kg.

Safety Action taken or intended by aircraft operator Jettime as a result of the event was noted as having included the following:

• Issue of an OM Bulletin ‘Flight Preparation Instructions’, which incorporated a procedure on when and how to update the OFP in the EFB, to ensure operation with current OFP data.
• An intention to improve EFB software to exclude the possibility of the valid OFP being hidden from the flight crew 
• The replacement of all older-type trailing edge solenoid control valves on their other 737 aircraft with the modified one.
• An intention to include information about the investigated event in pilot training with focus on application on NNC procedures and troubleshooting beyond the scope of an applicable NNC.

The Final Report was published on 13 June 2024. No Safety Recommendations were made.

Related Articles

• Fuel - Flight Planning Definitions
• Missed Approach
• Flaps
• Diversion
• Fuel - Diversion to Weather Alternate
• Low Level Windshear
• Non-Normal Checklist
• Fuel Emergencies: Guidance for Controllers