B752, vicinity Keflavik Iceland, 2013
B752, vicinity Keflavik Iceland, 2013
On 26 February 2013, the crew of a Boeing 752 temporarily lost full control of their aircraft on a night auto-ILS approach at Keflavik when an un-commanded roll occurred during flap deployment after an earlier partial loss of normal hydraulic system pressure. The origin of the upset was found to have been a latent fatigue failure of a roll spoiler component, the effect of which had only become significant in the absence of normal hydraulic pressure and had been initially masked by autopilot authority until this was exceeded during flap deployment.
On 26 February 2013, a Boeing 757-200 (TF-FIJ) being operated by Icelandair on a scheduled passenger flight from Copenhagen to Keflavik suddenly made an un-commanded roll left whilst on a night ILS approach at destination in VMC leading to a temporary loss of control and the declaration of an emergency. After recovery of control, a further approach was flown in abnormal configuration to an uneventful landing.
An Investigation was carried out by the Icelandic Transportation Safety Board. The FDR and CVR were downloaded and the data from both were used to assist the Investigation. It was noted that both pilots were experienced on the aircraft type and, after establishing the nature of the loss of control and their response to it, comment on their performance was minimal and the Investigation was focussed on the airworthiness origin of the event.
The event sequence was established from the recorded data. During the cruise, the First Officer had been PF and the centre AP had been engaged. Shortly before the top of descent, annunciations indicating a failure of the right hydraulic system had appeared on the Engine Indicating and Crew Alerting System (EICAS) and the appropriate QRH responses were completed. The crew were aware that one consequence of this malfunction would be that some roll spoilers on each wing would be inoperative and noted that although an increased landing distance would now be required, the runway length at their destination meant that this would not be a factor.
During the descent, a test of the roll spoilers found that they were "performing poorly" and an EICAS annunciation of 'SPOILERS' appeared. Radar vectors to an early establishment on the ILS for Runway 20 at Keflavik were requested; until on final approach, Instrument Meteorological Conditions (IMC) prevailed. Once established on final approach, flaps were extended and the landing gear selected down. When the flaps reached the 30° position at 7nm from touchdown and an altitude of 2400 feet, a vibration became apparent and an uncommanded roll to the left began. No new EICAS annunciations appeared.
The PF disengaged the autopilot and unsuccessfully tried to correct the heading manually and the commander called for a go-around. As this was commenced, the left roll continued, reaching a maximum of 34° within 8 seconds of the upset beginning. The commander selected the flaps from the 30° to the 20° position and the bank angle decreased to 18° and "when the flaps were somewhere between 20° and 5° the pilot felt the airplane responding to the right steering wheel input".
The aircraft exceeded the procedure go around stop height of 3000 feet due to the pre-occupation of the crew with regaining control (and because the missed approach altitude had not been set at the time the upset occurred) and was levelled off at 4000 feet, some 50 seconds after the upset had begun. Flap retraction was not complete before the airspeed exceeded the flap speed limit of 220 KIAS. The aircraft was then pitched down to descend to the correct go-around altitude of 3000 feet and the airspeed continued to increase during a rapid descent. The PF was aware of a high descent rate developing - up to 3500 fpm - but "did not immediately take the proper action to correct it". The commander declared an emergency to ATC and "at the same time flap retraction was stopped with the flaps in the 1° flap position, leaving the aircraft in a flap overspeed condition" - subsequently attributed to an oversight whilst busy.
TWR asked if they could reach the airport and the commander replied “Negative, we have problem with the controls... just a moment we are trying to regain control”. Following this response, ATC initiated the airport emergency response plan.
Almost two minutes after the initial upset, the aircraft was levelled off at an altitude of 3000 feet. Soon afterwards, the commander took over as PF and the First Officer, in response to an offer of runway 29, advised TWR (in Icelandic) “we’re going to start by regaining control of the airplane. We have lost the right hydraulic system and it behaves strangely. We are trying to regain control and would like to continue the present course if possible.”
It was noted that "three minutes and 22 seconds after the initial upset, the flight crew contacted ATC and advised that they had regained control". Preparations were made for a further approach to runway 20 and, after discussing the upset and concluding that it had to do with the flight controls and that it had occurred when the flaps were fully deployed, it was decided that only 15° flap would be used. The crew requested that a trauma team should meet the passengers after arrival. The track of the aircraft during the event is shown on the diagram below.
FDR data showed that there had been a pre-existing spoiler fault which had only begun to have an effect on the flight control system once the unrelated malfunction of the hydraulic system occurred. The spoiler fault was found to be attributable to a fractured housing of a valve in actuator No 6 which, without the presence of normal hydraulic system pressure, would have allowed hydraulic fluid to leak and thereby compromised the normal function of the actuator. The actuator Original Equipment Manufacturer (OEM) Moog advised that similar failures had been seen before during overhauls but had never featured as part of an in-flight incident and had not been reported to Boeing. The cause of the hydraulic system failure was found to have been a cracked hydraulic pipe, the suspected origin of the cracking being a weld flaw.
The combined effect of the two identified airworthiness malfunctions in creating the sequence that eventually led to a loss of control were as follows:
- a spoiler module failure present for the past 27 flights before the investigated upset had been caused by a latent failure in the actuator of the No 6 roll spoiler due to a cracked Blocking and Thermal Relief Valve Housing
- the spoiler failure would have been masked in service by normal hydraulic back system pressure
- the subsequent loss of right system hydraulic pressure combined with the pre-existing actuator fault caused the faulty spoiler to float
- the lateral asymmetry created by this was initially masked by the AP but shortly after the flaps were selected to 30º, the limit of AP corrective authority in this respect was reached and the sudden roll left occurred without the AP disconnecting.
The formally documented Causes were as follows:
- Latent failure in the actuator of spoiler #6, due to a cracked Blocking and Thermal Relief Valve Housing
- Loss of right hydraulic system pressure due to cracked hydraulic tube
- Full deployment of the flaps, with the right hydraulic system unpressurised and the Blocking and Thermal Relief Valve Housing cracked, caused spoiler #6 to float
- Floating spoiler #6 separated the airflow over a section of the left wing, causing the airplane to roll to the left due to an imbalance in lift between the left and the right wings
Safety Action already taken during the course of the Investigation was recorded as including the following:
- Icelandair carried out a fleet check for malfunctioning spoilers which resulted in four actuators being replaced on other aircraft.
- Moog redesigned the housing of the actuator blocking and thermal relief valve to improve its fatigue performance by substituting a stainless steel for aluminium
- Boeing communicated with 757 operators to appraise them of the Investigation findings and revised FCOM procedures in the event of un-commanded roll after a hydraulic system pressure loss and advised its intention to release on 25 June 2016, with reference to a corresponding Moog Component SB, an Alert SB for installation of the redesigned spoiler blocking and thermal relief valve.
Five Safety Recommendations were made as a result of the Investigation as follows:
- that Moog in co-operation with the airplane’s manufacturer, set up a program to support fleet wide replacement of the blocking and thermal relief valve housing with the fatigue improved unit made from stainless steel
- that Boeing issue the planned service bulletin 757-SB57A0154 to support fleet wide replacement of the Blocking and Thermal Relief Valve Housing in co-operation with the actuator‘s manufacturer
- that Boeing research other Boeing large transport category aircraft for similar spoiler actuator design and take corrective action as needed
- that the FAA research the need for making inspections, and possible replacement, of spoiler actuator’s Blocking and Thermal Relief Valve Housing mandatory via issue of airworthiness directive, for Boeing 753 airplanes
- that the FAA research the need for making inspections, and possible replacement, of spoiler actuator’s Blocking and Thermal Relief Valve Housing mandatory via issue of airworthiness directive, for other large transport category aircraft with similar spoiler actuator design
The Final Report was completed on 13 August 2015.
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