Swiss BCS3 at Porto on Jul 15th 2018, spoiler warning during takeoff run with insufficient thrust
Last Update: March 11, 2020 / 11:43:58 GMT/Zulu time
Incident Facts
Date of incident
Jul 15, 2018
Classification
Incident
Airline
Swiss
Flight number
LX-2077
Departure
Porto, Portugal
Destination
Geneva, Switzerland
Aircraft Registration
HB-JCC
Aircraft Type
Airbus A220-300
ICAO Type Designator
BCS3
Switzerland's SUST reported the occurrence was rated a serious incident, the SUST have opened an investigation.
On Mar 11th 2020 the SUST released their final report concluding the probable cause of the serious incident was:
The serious incident, in which a commercial aircraft took off with insufficient engine power, can be attributed to the fact that the flight crew was too late to notice that the engine power required for takeoff was not set.
The following factors contributed to the serious incident:
- Non-compliance with the aviation company's standard operating procedures (SOP);
- Inappropriate prioritisation by the flight crew during the takeoff roll.
Although they did not influence the development and course of the serious incident, the following risk factors were identified during the investigation:
- The design of the spoiler deployment;
- The design of the autothrottle (AT), whereby the AT switches to HOLD mode during the takeoff roll, even if the required takeoff power (target N1) has not yet been reached.
The crew decided for a takeoff with flaps in position and a derated take off thrust corresponding to the maximum possible thrust reduction. The crew thus computed V1=117 KIAS, Vr=122 KIAS and V2=129 KIAS. Engine thrust was calculated to 80.7% N1. The Accelerate-Stop Distance was computed at 1746 meters.
The SUST described the sequence of events:
At 01:59:36 UTC, the flight crew received clearance to line up and take off. When lining up, the commander (CMD) armed the autothrottle (AT). The flight crew performed a rolling takeoff. Aligned to the runway, the CMD advanced the throttles at 02:00:31 UTC, assuming that the AT would then set the takeoff thrust required to an N1 of 80.7 % (see chapter 1.6.3.2). The data shows that the throttle lever angle (TLA) was 20.6¡ã after the throttles had been advanced. For AT activation, the TLA would have needed to be more than 23¡ã; the AT therefore remained armed without being engaged. This went unnoticed by the flight crew.
The data also shows that, after the wheel speed (WS) had exceeded 60 kt during the takeoff roll (see chapter 1.6.3.2), the spoilers automatically extended without alerting the flight crew in the form of a warning (see annexes 1 and 2).
As per the standard operating procedures, the flight crew has to carry out the socalled ¡®80-kt check¡¯ at an indicated airspeed (KIAS) of 80 kt (see figure 9). That is to say, both pilots have to verify that the speeds displayed on their screens are identical and that the N1 rpm is equivalent to the takeoff power required. The data shows that, by this time, the power output of N1 was set at 65.3 % instead of the required 80.7 %. Both pilots stated that they could not say for certain whether they had executed the 80-KIAS check including the associated monitoring of the power output.
According to the CMD, acceleration seemed slower than usual at a speed of between 90 and 100 KIAS; however, he sensed a degree of uncertainty as takeoff was carried out at derated thrust. At that moment, the copilot notified the CMD that the FMS was working again and that the flight plan no longer displayed a discontinuity.
Due to the slow acceleration, the CMD checked the set takeoff power. He then realised that it was too low and pushed the throttles forward. The data shows that the speed was 109 KIAS at this time and that the throttles had been set at a TLA of 28.5¡ã, equivalent to an N1 power output of 76.6 %. According to the flight crew¡¯s statement, they became aware almost simultaneously of the CONFIG SPOILER warning, which illuminated for four seconds. The copilot responded to this warning at once, voicing that the spoiler lever was in the retracted (RET) position. The CMD briefly considered aborting takeoff. However, as he had exceeded speed V1 (117 kt) in the meantime, he decided to continue the takeoff roll. The aircraft took off 52 seconds after takeoff was initiated, at a distance that was 1.5 times the length of the calculated takeoff distance, and approximately 1000 metres before the end of the runway.
The remainder of the flight was uneventful. The aircraft landed in Geneva at 04:02:29 UTC.
The captain (57, ATPL, 14,766 hours total, 301 hours on type) was assisted by a first officer (34, ATPL, 1,783 hours total, 397 hours on type), in their statements after the occurrence both did not rule out fatigue.
The SUST describes the autothrottle system:
The aircraft HB-JCC is equipped with an autothrottle (AT) system that automatically manages the engine thrust. When the AT is engaged and not in AT HOLD mode, the AT servomotors in the throttle quadrant assembly (TQA) automatically position the thrust levers during the complete flight profile. The AT also ensures engine synchronisation.
The AT is normally engaged manually and can be disengaged at any time. The AT is highly integrated with the automatic flight control system (AFCS) and the flight management system (FMS).
The AT also provides speed and thrust envelope limiting. The AT system status is displayed on the flight mode annunciator (FMA) and fault messages are displayed on the EICAS page.
Before takeoff, the AT is armed by pressing the push button (p/b) on the flight control panel (FCP). The dark line above the AT p/b illuminates in green and the FMA shows AT (white) (see figure 6). The white arrow indicates that the AT uses navigation sources and altimeter settings of the left side.
At takeoff, the thrust levers are advanced to approximately N1 of 50 %. Once the engines are stabilized the thrust levers are moved further forward. If the throttles are further advanced through the 23-degree Thrust Lever Angle (TLA) position (approximately 60 % N1), the AT activates and takes over thrust lever control, and the thrust levers automatically travel into the takeoff thrust position. THRUST and AT are now displayed in green and SPD (speed) is shown in white (armed). Furthermore it has to be mentioned that the TLA value is not visible to the flight crew.
When the airspeed increases above 60 KIAS, the THRUST HOLD mode activates. In HOLD mode, all autothrottle commands are suspended and power is removed from the thrust levers. This means that the thrust levers and N1 speed remain at this approximate position. Normally, this position corresponds to the takeoff N1 selected through the FMS.
Since the TLA in the present case was still less than 23¡ã, the AT remained armed and switched directly to HOLD mode when the speed exceeded 60 KIAS.
The THRUST HOLD mode remains active until the aircraft reaches an altitude of 400 ft above ground level.
The SUST described the ground spoilers:
The ground spoilers operate symmetrically to provide lift dumping at touchdown. Automatic deployment of the ground spoilers is computed through the primary flight control computers (PFCC). Amongst other things, the deployment logic is based on a wheel speed (WS) of 60 kt and on the weight-on-wheels. Furthermore, the thrust lever angle (TLA) must be less than 23 degrees. The lift dump function is automatic and does not need to be armed by the pilot.
Conversely, if the TLA of both throttle levers is less than 23 degrees during aircraft acceleration, the spoilers will deploy automatically if the wheel speed is above 60 kt.
A respective note can be found in the checklist ¡°touch and go landing procedure¡± (FCOM Operational Guidance, chapter 08.02.02 page 7). The first note reads:
¡°Ground lift dumping (GLD) are automatically deployed with weight©\on©\wheels, and retract when the thrust levers are advanced beyond a 23©\degree lever angle (approximately vertical).¡±
The SUST analysed:
The fact that the spoilers extended during the takeoff roll was due to their design by the manufacturer. Prerequisites for spoiler deployment are amongst others a wheel speed (WS) of more than 60 kt and a throttle lever angle (TLA) of less than 23¡ã (see chapter 1.6.3.2). Both conditions are met during a landing, a touch and go and an aborted takeoff because the throttles are in the idle position at this moment.
During this serious incident, these conditions were also met because the TLA was just 20.6¡ã at the start of the takeoff roll. The spoilers therefore extended when the WS of 60 kt was exceeded. This change in configuration is only shown on the flight control (FLT CTRL) synoptic page and therefore cannot be seen by a flight crew as this page is not preset on any multifunction window (MFW) during the takeoff roll (see chapter 1.6.2). These windows furthermore display the deflections of the spoilers in green, which relates to a normal condition, and this therefore does not necessarily signal a warning.
A spoiler deployment design that does not consider the acceleration vector only meets the criteria as given for an aborted takeoff, a landing and a touch and go.
Therefore, information about the operation of the spoilers can only be found in the FCOM in relation to these aforementioned flight phases. The fact that the spoilers also extend without the flight crew intending or noticing during a takeoff roll, in which the critical TLA of 23¡ã is not exceeded, is not stated anywhere. In addition, a TLA equivalent N1 of approximately 60 % is specified in the section that describes the AT (FCOM page 03-05-12), but an N1 of 68 % is specified in the TAKEOFF CONFIGURATION WARNING section (FCOM page 08-05-40).
The above facts lead to the conclusion that this scenario ¨C which was not an isolated incident (see chapter 1.18.1) ¨C was not considered when the spoiler deployment was designed. Yet, the fact that the spoilers can deploy during the acceleration phase of the takeoff roll without intervention from the flight crew carries great safety risks. A safety recommendation was therefore issued (see chapter 4.1.1).
Only once the CMD had pushed the throttles forward at a later time did the TLA change to above 23¡ã and the spoilers retracted within 2 seconds, 23 seconds after having extended (see annex 1). As observed by the flight crew, the SPOILER CONFIG warning displayed on the EICAS for 4 seconds whilst the spoilers were retracting.
The AT remained armed and switched directly to HOLD mode, when 60 KIAS was
exceeded. The power set at that time remains the same until the AT switches to THRUST mode at an altitude of 400 ft above ground. Any power not matching the required takeoff power (target N1) represents a high safety risk if there is no intervention from the flight crew (see chapter 1.18.2).
When the PF pushed the throttles forward to a TLA that exceeded the critical 23¡ã at a speed of 109 KIAS, the AT should have disconnected automatically, and the crew should have been informed about this by an aural AUTOTHROTTLE alert (see chapter 1.6.3.2). Why the AT did not disconnect explained the Transport Canada Civil Aviation (TCCA) National Aircraft Certification (NAC) by the fact that the AT switched directly from the ARM to the HOLD mode and therefore could only get active after passing 400 ft above ground level (cf. footnote 10). However, this is in contrast to the incident of HB-JCF mentioned in chapter 1.18.1 and the statement in the All Operator Message (AOM) dated 21 December 2018 and published by the manufacturer (cf. chapter 4.3.2).
The SUST analysed the human factors:
When lining up on runway 35, the PF armed the AT. It can be assumed that the AT was armed following the closed-loop procedure described in OM A and that the PM checked the action taken by the PF.
After the flight crew had received clearance to line up and takeoff, the PF advanced the throttles. The data shows that the throttles stopped at a TLA of 20.6¡ã. The TLA of 23¡ã required to engage the AT was consequently not exceeded. As a result, the AT was not engaged and the white THRUST indicator on the FMA did not change its colour to green. During this phase, closed-loop communication obviously did not take place as intended, as the flight crew did not notice that the AT only remained armed and had not been engaged.
The fact that the deployment of the spoilers went unnoticed by the flight crew can be explained by the prescribed choice of displays during takeoff (see chapter 1.6.2). This would only have been visible on the appropriate synoptic page, however the indicator for extended spoilers would have been displayed in green, which generally does not imply that they are in a wrong position.
In line with the operating procedures, the flight crew had to check the speed and engine data for takeoff at a speed of 80 KIAS (see chapter 1.17.2.3). Both pilots stated that they could not remember whether they had performed this check. This cannot be explained by anything other than that the check ¨C if it took place ¨C was not performed with the required level of attention. Both the FMA indicator as well as the N1 engine data ¨C set at only 65.3 % instead of 80.7 % ¨C did not correspond with expectations. At a speed of between 90 and 100 KIAS, the PF became aware of the unusually low level of acceleration, noticed that the power output did not meet the required takeoff power, and advanced the throttles further. The data shows that a TLA of 28.5¡ã was thereby reached, which is only equivalent to an N1 rpm of 76.6 % and was still below the calculated takeoff power. The fact that this lack of required power continued until the AT automatically changed modes at an altitude of 400 ft above ground suggests that the flight crew was unaware of the power actually set.
In light of the remaining length of the runway and the required obstacle clearance along the takeoff path, in particular in the event of engine failure, it is advisable in a situation such as this that the thrust levers are pushed forward to the mechanical stop. This would cause the AT to disconnect and the appropriate warnings to be triggered, which would however result in maximum possible takeoff thrust being initiated (see chapter 1.6.3.2). This is particularly relevant during a takeoff with derated takeoff thrust.
When the CONFIG SPOILER warning was displayed, the PM immediately checked that the spoiler lever was in the retracted position and confirmed this to the PF. This reaction is understandable, as the flight crew had ¨C as is reasonable ¨C not anticipated the spoilers to deploy automatically during a takeoff roll. The PF briefly considered aborting takeoff but dismissed this thought as the decision speed V1 (117 kt) had already been exceeded in the meantime. According to the data, the CONFIG SPOILER warning displayed one second after the throttles had been advanced and disappeared four seconds later. The decision not to abort the takeoff was appropriate for the situation because, when aborting a takeoff after V1, it can never be said with any certainty whether an aircraft can be brought to a stop on the remaining length of the runway.
Aircraft Registration Data
Incident Facts
Date of incident
Jul 15, 2018
Classification
Incident
Airline
Swiss
Flight number
LX-2077
Departure
Porto, Portugal
Destination
Geneva, Switzerland
Aircraft Registration
HB-JCC
Aircraft Type
Airbus A220-300
ICAO Type Designator
BCS3
This article is published under license from Avherald.com. © of text by Avherald.com.
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