On 1 December 2021, a lightly loaded Boeing 737-800 (G-JZHL) being operated by Jet2.com on an international non-revenue positioning flight from Kuusamo to London Stansted only became airborne in night VMC shortly before the end of the 2,500 metre-long runway and failed to climb normally until the correct thrust setting was applied. Thereafter, the flight was completed without further event.
The event was reported by the aircraft operator to the UK CAA and the UK AAIB. The latter then contacted the Safety Investigation Authority of Finland, which delegated the conduct of the Serious Incident Investigation to the UK as State of the Operator and a Field Investigation was carried out.
Relevant recorded data was downloaded from both the FDR and the QAR but such data on the 2 hour CVR would have been overwritten prior to the aircraft completing its flight to the UK. No relevant radar recordings were available for Kuusamo airport and the airport’s RTF recording equipment was found to have been inoperative for in excess of two months prior to the investigated event (although it was subsequently repaired eight days after the event had occurred).
It was noted that the 44 year-old Captain had a total of 15,490 hours flying experience which included 3,344 hours on type and the First Officer had a total of 4,582 hours flying experience which included 2,399 hours on type. Neither pilot had operated to/from Kuusamo before. It was also noted that although Kuusamo Airport sometimes provided ATC service, this provision was dependent on the expected traffic level and ATC service there had been downgraded to a FIS shortly before the aircraft landed after its inbound flight. The FISO on duty had ten years’ experience in that role and had recently completed simulator recurrent training in this role.
The investigated departure followed an earlier inbound passenger flight operated by the same aircraft and crew. It was decided that the First Officer would act as PF for the positioning return flight and determined that the Takeoff Weight would be 52,100 kg (the MTOW was 70,530 kg) so both pilots would have been expecting a greater than usual acceleration during the takeoff roll. Although the inbound aircraft had landed on runway 30, departure was planned from runway 12 - both directions involving a 5 knot crosswind component.
The runway-related conditions for departure were unchanged from those for the earlier landing with a covering of 3mm of dry snow, an air temperature - 8°C and a light wind being reported. This meant that RWYCC 5, defined as “braking deceleration is normal for the wheel braking effort applied and directional control is normal”, was applicable although contrary to the specified RWYCC criteria, the airport was reporting RWYCC 4. This made no difference to operational requirements and a de-rated takeoff thrust of 89% N1 was set.
However, although the runway was not contaminated, the airport apron was, so it was decided to taxi with the flaps up and select them only when about to line up on the runway, the beginning of which was almost abeam of and very close to the apron. Because of this apron contamination, standard operating procedures required that a pre-takeoff engine run-up to 70% N1 for 30 seconds against the brakes should be carried out once lined up on the runway to ensure that any ingested ice was cleared from the engines.
To comply with the operator’s general operating procedure (First Officers may not taxi the aircraft), the Captain taxied the aircraft the very short distance from the apron to the runway before handing control to the First Officer once the aircraft was lined up on the runway with the required flaps 5 set. On being informed that the flight was ready for departure, the flight information service officer (FISO) advised that the runway was clear and gave the surface wind.
The First Officer then began to advance the thrust levers towards the required 70% N1 whilst holding the toe brakes and the Captain reported having selected the secondary engine instruments on the lower centre screen and held one finger over the relevant button ready to clear the display after the run-up was complete and also prepared to start his timer to achieve the required 30 second run-up once the run-up thrust was reached.
The First Officer stated that as he continued to increase the thrust towards 70% N1, the aircraft had begun to “slide and yaw”. This possibility, although unlikely on an uncontaminated runway, had been briefed and that if it occurred, the brakes would be released and the takeoff continued which the Captain confirmed when the unexpected movement began. Having released the brakes following the Captain’s confirmation, the First Officer concentrated on directional control but failed to press the TOGA button before immediately removing their hand from the thrust levers, an action intended to allow the Captain to place their hand on them until V1 in case of a decision to reject the takeoff.
The Captain did not notice this omission having prioritised his response to an almost concurrent call from the FISO asking for confirmation that they intended to turn right after takeoff to which he responded in the affirmative. Crucially, the Captain also did not make the mandatory check that takeoff thrust was correctly set and also did not make the mandatory “thrust set” standard call, an omission unnoticed by the First Officer.
As the aircraft approached 80 knots, the Captain stated that he had been “aware that something was not right” but could not resolve what it was. He considered that the acceleration was “slightly low” but thought that this might be due to (unreported) runway surface contamination. The crew made the normal calls at 80 knots without further comment but on reaching the 93 knot V1 and continuing, both pilots stated that they had been aware that “something was not right”.
On attempting to initiate rotation at VR, the First Officer recalled finding that the aircraft was “very heavy” in pitch and as he looked down to the PFD he saw that there was no vertical command displayed and had therefore focused on flying a pitch attitude to maintain airspeed. The achieved airspeed was “hovering around V2” which he described as “like flying an engine failure on takeoff in the simulator” and announced “we need more power”. At this prompt, the Captain finally noticed that takeoff thrust had not been set and manually advanced the thrust levers to 89% N1. Once at 2,400 feet QNH (approximately 1500 feet aal) thrust was reduced and after a further 1,500 feet, acceleration and flap retraction was commenced. Once clean, ‘LVL CHG’ mode was selected to continue the climb which resulted in the restoration of the FD vertical command indicator and the remainder of the flight was completed without further event.
An examination of the FDR data showed that although the maximum system braking pressure was the standard 3,000 psi, the First Officer had only applied a pressure of “600-700 psi” before beginning to increase the thrust from idle towards the agreed engine run-up setting so that 12 seconds later, as the engine speeds approached 70% N1, the aircraft began to move forwards. At a recorded groundspeed of 6 knots, a radio transmission was made from the aircraft, followed by another one at a recorded 53 knots airspeed. Passing 80 knots, the acceleration was approximately 2.25 knots/second. V1 (93 knots) was reached 32 seconds after brake release and after rotation was begun at VR (122 knots). It was then a six further seconds before the aircraft became airborne 400 metres before the end of the 2,500 metre-long runway and began climbing at 600 fpm at the reported indicated airspeed of around V2 (131 knots). Despite gear retraction and a reduction in pitch attitude, no further acceleration was recorded until the thrust levers had been advanced to the 89% N1 takeoff setting at 250 feet agl when approximately 1400 metres beyond the end of the runway as a right turn was commencing.
Although a rejected takeoff at V1 would have been achievable, had an engine failure occurred after V1, the aircraft would have been unable to get safely airborne with the 70% N1 thrust on the remaining engine.
An annotated plan view of the takeoff and second segment climb. [Reproduced from the Official Report]
Why It Happened
The initiating factor of a takeoff with insufficient thrust set was the failure of the First Officer to use adequate brake pressure for the engine run up. Boeing advised that even assuming RWYCC 4, the relatively light weight of the aircraft and the carbon brakes fitted to it, the brake pressure required to prevent the wheels rotating during the engine run up would have been in the range 1000-2000 psi dependent on the brake temperature and, to a lesser extent, the humidity. They also advised that in such conditions, the aircraft weight and CofG made it “unlikely that the tyres would have slipped on the runway surface”.
The First Officer’s resulting surprise at the unexpected movement he detected was followed by a failure of both pilots to follow the normal standard takeoff procedure. The Captain had been distracted from his primary command responsibilities by engaging in non-critical radio communications. What was not possible for the Investigation to explain was why it took so long for either pilot to identify why the takeoff roll and initial climb performance were ‘not right’ until 250 feet agl.
In respect of the First Officer’s use of inadequate brake pressure, it was noted that after being given an opportunity in the simulator to experiment with the brakes, he had “found the brake pedals have significantly more travel than he had previously been using”. An operator review of their historic OFDM data also found two events where it was likely that insufficient brake pressure had been applied during static engine run-ups.
In respect of the radio communication between the FISO and the Captain as takeoff was commencing, it was noted that a belated request was made by the former for confirmation of the direction of turn after takeoff. She had been expecting to receive this when the flight advised it was ready for departure but had not immediately requested when it was not mentioned in their call. The FISO reportedly “felt that she was required to clarify (the flight’s) intentions” despite being aware of ICAO operational safety procedures stating that a FISO is not allowed to “issue any instructions to an aircraft during takeoff” so could not have done so had the crew not confirmed a right turn.
It was found that a Finnish AIP requirement for “aircraft with a MTOW exceeding 2000 kg to make a right turn after taking off from runway 12 at Kuusamo" was included in the aircraft operator’s IFR chart information used by the crew and noted that the flight had already been cleared to a waypoint to the south of the airport. However, another AIP requirement covering operations at airports with only FIS which was unknown to the pilots and their operator was found to state that any intention to turn right after takeoff must be “reported to the AFIS unit”.
In respect of type-experienced pilots’ ability to perceive slower than normal acceleration, the 2.25 knots per second acceleration rate at 80 knots during the Kuusamo takeoff was compared with the distribution of acceleration rates recorded at that speed in a sample of 73,669 Boeing 737-800 takeoffs covering a range of OAT and airport elevations. This found that the Kuusamo takeoff acceleration was significantly below 99.7% of them, supporting a conclusion that visual cues for slow acceleration do not differ enough for pilots to detect it until they are presented with “an atypical visual scene” such as the end of the runway approaching”.
The AAIB noted that its investigation of another similar low-thrust late-takeoff by an aircraft of the same type at Belfast International in 2017 had, although it had different origins to those to those in this event, also involved a lack of perception of abnormally slow acceleration by the pilots involved and had been the basis for a Safety Recommendation on the need for ‘Takeoff Acceleration Monitoring Systems (TAMS)’ which had not yet been followed by any definitive action.
As part of the current Investigation, a non-exhaustive list of 32 takeoff performance events worldwide for the period of just under five years since mid-2017 was compiled and included in the Official Report as an Appendix. These events - which had a range of primary origins and consequences - included (in addition to the already-mentioned Belfast event) the following:
The formally-stated Conclusion of the Investigation was as follows:
The aircraft took off with insufficient thrust set because the TOGA button was not pressed. It was not pressed because the [First Officer] was startled by the aircraft moving as he commenced the run-up against the brakes. The aircraft started to move because insufficient brake pressure was applied. Human checks designed to detect the insufficient thrust were ineffective because both pilots were attending to other tasks. The commander was responding to a radio call from the FISO during the start of the takeoff roll. Neither pilot detected the low thrust until after the aircraft was airborne.
The AAIB and other Safety Investigation Agencies have investigated numerous previous takeoff performance incidents, and this incident provides further evidence that the current barriers in place to prevent such incidents are not always effective.
Safety Action taken prior to completion of the Investigation by Jet 2 was noted as having included, but not necessarily have been limited to, the following:
- Enhanced OFDM procedures were introduced to detect any further instances of inadequate brake pressure application prior to static engine run-ups.
- The inclusion of a static engine run-up in the next recurrent simulator check for all pilots to include ‘some distraction’ during the start of the takeoff roll.
- Amended their FCOM procedure for pre-takeoff engine run-ups.
- Added a requirement to the OM-C to notify before commencing takeoff the intended direction of turn after takeoff for all FIS departures from Finnish airports.
Two Safety Recommendations were made as a result of the Investigation as follows:
- that the UK Civil Aviation Authority, in conjunction with other regulatory authorities, develop a set of technical specifications and, subsequently, develop certification standards for an on-board system that will alert the crew of an aircraft to abnormally low acceleration during takeoff. [2022-018]
- that the UK Civil Aviation Authority encourage all UK Air Operator Certificate holders to implement into their flight data monitoring programme algorithms to detect the precursors relevant to the monitoring of takeoff performance detailed in the European Operators Flight Data Monitoring Document, Guidance for the implementation of flight data monitoring precursors.
The first of these Recommendations was a restatement of a similarly worded Recommendation [2018-014] made as a result of the investigation into the 2017 Belfast Serious Incident referred to above which was directed to EASA in conjunction with the FAA prior to the UK leaving the EU.
The Final Report of the Investigation was published on 6 October 2022.