Iran Air A306 at Stockholm on Jan 16th 2010, went off runway

Last Update: January 17, 2013 / 19:19:37 GMT/Zulu time

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Incident Facts

Date of incident
Jan 16, 2010

Classification
Incident

Airline
Iran Air

Aircraft Type
Airbus A300

ICAO Type Designator
A306

The Swedish Havarikommission (SAIB) released their final report concluding the probable causes of the incident were:

Operational

- Deficiencies in the certification process for large aircraft with wing-mounted engines with regard to requirements for yaw stability in the event of sudden loss of engine power in the speed range below VMCG.

- Deficiencies in the pilot training with regard to training for sudden losses of engine thrust in the speed range below VMCG.

Technical

- Deficiencies in the approval and follow-up of the Dabbler TIG Weld repair on the engineÂ’s diffuser aft air seal.

The SAIB analysed that the crew undertook a routine takeoff. About 10 seconds into the takeoff, at a speed of 54 knots, one or more edges in the diffuser aft air seal separated leading to the engine failure and loss of thrust. The crew just had heard a muffled bang and had no indication in the cockpit when the aircraft began to veer left. On the contaminated runway surface the nose wheel was not able to generate sufficient force to counteract the yaw moment induced by the thrust of the right hand engine, that remained at full takeoff thrust for about 1.5 seconds after the left hand engine had suddenly lost power. The crew did not apply differential braking in the correct direction.

Although the first officer retarded the thrust levers just over one second after the bang, applying braking and right rudder at the same time, the veer could not be corrected and the aircraft went off the runway. The SAIB analysed that even if the first officer had applied reverse thrust as recommended by the manuals, the aircraft may have run off the runway.

The SAIB analysed: "The forces from the moment in combination with the partly contaminated and slippery surface, probably meant that the pilots had little chance of altering the sequence of events in any decisive manner without the contribution of forces from asymmetric braking in the opposite direction. The moment from the asymmetric braking in the “wrong” direction has probably also contributed to the excursion to a degree, which has not been possible to determine."

The diffuser, installed at the stage 1 high pressure turbine shaft with an interference fit, receives a small quantity of air from the high pressure compressor stage 14. The air is being used to cool hot parts of the engine and to control the axial pressure balance of the rotor system. Seal parts control and regulate the air flow.

When the aft seal broke, the debris entered the core flowpath in addition to the loss of pressure balance, which caused the engine to stall. The debris caused further damage to the high pressure turbine (HPT) guide vanes, HPT blades, low pressure turbine (LPT) blades and LTP guide vanes resulting in the loss of fan speed (N1) and a rise of the exhaust gas temperature (EGT).

The aft seal had been installed on a different engine until Oct 10th 2002, then was removed for an overhaul. The edges of the seal were repaired for a first time, the seal was then mounted on a second engine on Nov 28th 2002, the seal was removed from that engine on Jan 5th 2007. Measurements showed the edges of the seal teeth were below required nominal dimensions, but no welding was needed. The seal was repaired by surface treatment. The seal returned to service on Mar 2nd 2007 on a third engine. That third engine suffered foreign object damage a few months later, the aft seal was removed for inspection. On Aug 31st 2007 the aft seal was installed on the incident engine and remained there until the incident.

At the time of the investigation 4 cases of aft seal failures were known with EP-IBB being the third case. All four failed aft seals had undergone repair by methode "Dabber TIG-Weld repair" approved by the engine manufacturer.

During the investigation, in September 2011, it became known that there had been two more cases unknown to the engine manufacturer, these two cases too included aft seal repairs applying the Dabber TIG-Weld repair.

The SAIB analysed that the event most likely was caused by fatigue cracks originating in the Diffuser Assembly Aft Air Seal teeth parent metal to dabber TIG weld interface. Subsequently 9 bolts securing the rotating diffuser assembly sheared permitting the aft seal to separate and travel radially permitting the debris to enter the cavity in front of the HPT stage 1 disk. The debris then struck the face of the HPT stage 1 disk causing substantial damage to the HPT stage 1 blades, guide vanes, bolts and blade retainers, the increased debris "working its way" downstream causing substantial damage to all downstream turbine stages.

The SAIB stated: "The overall assessment of the investigation results suggests that the fatigue had started in the Dabber TIG Weld repaired aft air seal tooth, at the interface of the seal parent metal to weld material. The known cases of engine failure caused by the separation of the diffuser aft air seal have all shared the common factor of having been Dabber TIG Weld repaired on one or more occasions. In an engine event which results in liberated debris entering the engine gaspath, consequences in the form of additional downstream damages are great.

In light of what has been reported, SHK draws the conclusion that the current procedure for repairs of the engine part in question should be called into question."

The SAIB stated, that although there are regulations concerning directional stability in case of an engine failure above minimum ground control speed Vmcg, there are no such regulations for speeds below Vmcg. Vmcg defines the minimum speed at which directional control and stability can be maintained in case of an engine failure at takeoff thrust.

The SAIB analysed: "The conclusions that can be drawn from the investigation are that sufficient frictional forces at the nose wheel cannot be generated in the lower speed range on a runway with reduced friction. In the event of a loss of engine thrust on a wet or contaminated runway, a yaw moment will turn the aircraft towards the side of the malfunctioning engine. The nose wheel will – irrespective of the displacement angle – skid, i.e, slide over the surface with a direction that deviates from the aircraft’s longitudinal axis." The SAIB further analysed that although in simulator tests it had been able to keep the aircraft on the runway the accuracy of the simulator models for main and nose wheel tyre forces could not be substantiated. The SAIB thus views "the simulator tests as very interesting from a broad perspective, but considers at the same time that in the present case it has not been possible to recreate the actual sequence of events with sufficient accuracy. Probably the only way to achieve this would be to carry out all testing “for real” in an aircraft."

The SAIB continued with regards to aircraft certification that "Failure to set requirements concerning requirements for the manoeuvrability of aircraft in the event of a sudden loss of engine power for the entire take-off sequence, including the most critical stage, is to accept a risk which according to SAIB is not in line with reasonable safety requirements for commercial aviation."

In the analysis of generic risks the SAIB stated: "In the incident, the excursion took place at a speed of approximately 60 knots, i.e. just over 110 kph. There is a considerable build-up of kinetic energy when a mass of 148 tonnes is travelling, out of control, at a speed of 110 kph. In this case, there were no serious consequences, but with just marginal displacements of the time, the incident could have had much more serious consequences. Had the incident occurred just a few seconds later, the aircraft would most probably have run onto taxiway Y9. At the holding point on this taxiway, aircraft are often lined up awaiting take-off. On average, 40% of all take-offs from runway 19R are carried out from the intersection at taxiway Y9."

As result of the investigation the SAIB issued recommendations to ICAO, FAA and EASA to review and enhance aircraft certification criteria concerning safety of aircraft throughout the entire takeoff sequence of an aircraft, in particular directional stability. In addition the SAIB recommended the FAA should review and revise the "Dabber TIG Weld repair method".
Incident Facts

Date of incident
Jan 16, 2010

Classification
Incident

Airline
Iran Air

Aircraft Type
Airbus A300

ICAO Type Designator
A306

This article is published under license from Avherald.com. © of text by Avherald.com.
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