Saudi MD90 at Riyadh on May 8th 2009, veered off runway during turn off
Last Update: June 18, 2024 / 18:34:16 GMT/Zulu time
Incident Facts
Date of incident
May 8, 2009
Classification
Accident
Airline
Saudi Arabian Airlines
Departure
Jeddah, Saudi Arabia
Destination
Riyadh, Saudi Arabia
Aircraft Registration
HZ-APW
Aircraft Type
McDonnell Douglas MD-90
ICAO Type Designator
MD90
A spokesman for the airport said, that the airplane skidded to the right after landing on runway 15L and went across taxiway G4 before coming to a stop with the nose gear on soft ground and the airplane settling on its left wing.
According to sources within the airline the airplane is considered to be write off due to the damage received, especially due to the wing damage.
On Jun 18th 2024 The Aviation Herald was able to obtain the final report by Saudi Arabia's GACA (General Authority of Civil Aviation) concluding the probable causes of the accident were:
Cause Related Findings
- The Captain decided to land with manual ground spoilers when the auto ground spoiler system was fully operational.
- The initiative by the Captain to conduct this improvised exercise contravened the Standard Operating Procedures (SOPs) and the Flight Operations Policy Manual (FOPM).
- The auto ground spoiler system was disarmed prior to landing.
- The spoiler/speed brake lever was partially applied manually after landing.
- The spoiler/speed brake lever was released before it was fully extended and latched.
- The spoiler/speed brake lever automatically retracted as per design.
- The ground spoilers never fully deployed.
- The loss of lift and aircraft deceleration were greatly reduced by the lack of ground spoiler deployment.
- Brakes were not used in an attempt to control or slowdown the aircraft.
- The Captain applied a large right rudder input with the intention of directing the aircraft onto the G4 taxiway exit.
- The aircraft exited the runway at high speed and was travelling too fast to successfully negotiate the right turn onto the G4 taxiway.
Other Findings
- A successful emergency evacuation was completed.
- The aircraft was properly certificated and had been maintained in accordance with approved procedures.
- The flight crew possessed the appropriate flight certification.
- The meteorological conditions were not a factor in this accident.
- The aircraft was dispatched with one MEL item not related to this occurrence.
- The auto ground spoiler system failure procedure was not clearly documented in neither the MD 90 Flight Handbook, nor in the Quick Reference Handbook.
- The Saudi Arabian Airlines Computer Based Training (CBT) did not include failure of the auto ground spoiler system as part of Abnormal Procedures Training for the MD-90 Flight Control Systems.
- Neither the initial, nor the recurrent flight simulator training included manual spoiler deployment during rejected takeoff and landing.
- “Spoiler” or “No Spoiler” callout methodology in the case of manual spoiler deployment was not clearly established or documented.
- The auto brake system was not armed for landing.
- Nose wheel steering was not used at any time during the landing.
- All four brake pressure sensors did not provide data to the DFDR.
- The CVR and DFDR had not been inspected in accordance with
ICAO/GACA Annex 6.3.12.
The GACA analysed that there was no evidence of any flight control system failure during landing. With respect to the actions of the flight crew the GACA wrote:
The sequence of events that led to this accident can be traced back to the decision of the Capt. to disarm the auto ground spoiler system before landing.
According to the SVA MD-90 SOPs, arming the auto ground spoiler system prior to landing was a requirement. This action was not optional or subject to pilot discretion. As such, this initiative by the Capt. to conduct this improvised exercise contravened the SOPs.
Further, the Capt. did not follow the rules of conduct stipulated in the Flight Operations Policy Manual (FOPM) by performing prohibited exercises, by not utilizing all appropriate checklists and by not complying with all the operating procedures outlined in the relevant company manuals.
The F/O did not express an opinion as to whether he supported or not, the decision of the Capt. to apply a non-standard procedure. His lack of experience (5 months since release) may have contributed to a reluctance to play a critical role in the decision making process.
The decision by the Capt. to apply a MEL operational procedure when the A/C auto ground spoiler system was functioning normally, started a chain of errors that eventually resulted in the high speed runway excursion via the G4 taxiway.
The MD-90 MEL contained an operational procedure that allowed the use of manual spoilers in the case of failure of the auto ground spoiler system. This MEL procedure was in itself, inherently safe. As such, the Capt. elected to practice a safe procedure while there were no passengers on board so that he would be better prepared to use the same procedure, if required.
During his interview, he explained that; since it was a ferry flight and that there were perfect runway and weather conditions at the time, there existed an ideal opportunity to “see the effect” of landing with manual spoilers. Although, his desire to improve his skill level and experience should be commended, his methodology put him in a situation of increased risk. A simulator is a more appropriate tool to practice abnormal procedures.
The Capt.’s decision was not challenged by the F/O. The F/O did not enter into a discussion with the Capt. whether such actions were appropriate or whether he agreed or not with the decision of the Capt. to apply this nonstandard operational procedure.
The activation of the speed brake configuration warning on touchdown did cause a certain level of confusion in the cockpit, particularly when the F/O had already verbally confirmed spoiler deployment.
The additional aileron control inputs required during the landing and the need for the application of the thrust reversers caused the Capt. to release the speed brake lever before it was fully extended. Without being fully latched in the extended position (fully aft and upwards), the lever automatically returned to the retracted position and the ground spoilers immediately restowed.
Spoilers are critical during landing as they create two beneficial effects. First, they greatly reduce the lift being generated by the wing and secondly, they increase drag, helping to slow down the A/C. The reduction of lift also transfers more load from the wing to the main gear, greatly improving braking and directional control. In this instance, it was determined that after the nose wheel touched down, the wing was still generating about 80,000 lbs of lift. By contrast, if the ground spoilers had been deployed, the wing would have been generating less than 40,000 lbs of lift. This would have increased the load on the main landing gear from around 27,000 lbs to over 67,000 lbs.
Directional control was heavily dependent on the frictional forces acting between the runway surface and the A/C tires. This force was derived by multiplying the weight of the A/C by the coefficient of friction. The greater the weight on the main wheels, the greater the main wheels retardation or cornering force capability. When a limiting amount of friction is available and it is all used for cornering, then little or no friction is available for braking and vice versa. The less the weight on the main wheels, the more the A/C ability to brake and/or corner.
In this instance, the A/C was at a relatively light weight and the ground spoilers were not deployed after touchdown, minimizing the normal forces acting on the main landing gear. This reduced ability of the A/C to brake or corner resembled slippery runway conditions, even though the A/C was on a dry runway surface. As there were no passengers on board and only the landing fuel, the light A/C weight further reduced the frictional forces.
When the ground spoilers re-stowed, there was no aerodynamic disruption of the laminar flow over the wings. With no spoiler drag or disruption of lift, most of the A/C weight was supported by the wing and not the main gear.
The lack of sufficient frictional contact with the runway surface explains the Capt’s difficulty in maintaining directional control of the A/C. During his post accident interview, the Capt. described the directional and roll control difficulties during the landing and explained how he used large rudder and aileron inputs in an attempt to maintain control.
Considering the highly swept wing of the MD-90, the application of rudder in ground effect would have also caused a roll tendency due to the secondary effect of yaw. Similarly, the application of aileron inputs would have also activated the roll spoilers, further exacerbating the situation.
Furthermore, when the nose wheel strut was compressed on landing, any rudder inputs would have caused a proportional nose wheel deflection through an arc of up to 13 degrees left or right of center. This fact would also have a significant effect on maintaining directional control.
A left rudder input occurred just prior to nose wheel touchdown. This left rudder input caused the initial change in the A/C’s heading to the left. It was also consistent with the left rudder and aileron applications as described by the Capt. after touchdown.
As the nose wheels made contact with the runway surface, a right application of rudder occurred for about two seconds. This began the initial divergence of the A/C to the right at a rate of around 3 degrees per second. This sweeping turn to the right was described by both FDC during their post accident interviews.
Both FDC described full left rudder pedal application after nose wheel touchdown. Furthermore, the F/O stated that he also applied full left rudder in order to assist the Capt. to restore directional control of the A/C. The DFDR data did not show either full left rudder pedal input or full rudder trailing edge deflection to the left. When rudder pedal application was compared to rudder trailing edge response, they were directionally proportional. At no time was the left rudder pedal fully applied. Consequently, at no time did the rudder fully deploy to the left.
There were 2 small left rudder applications over the next 6 seconds which reduced the rate of turn from 3 to 2 degrees per second to the right. These rudder inputs were not large enough to arrest the A/C rapid sweeping turn to the right of the runway centerline.
The MD-90 had an auto braking system (ABS) that apply the brakes automatically during landing without any brake pedal deflection. However, auto brake application would only occur if the auto brake system was armed before landing. The Capt. elected to land the A/C without auto brakes.
The standard operating procedures (SOPs) described in the Normal Procedures Chapter of the SVA MD-90 Flight Hand Book (FHB) allowed the FDC the option of a landing without auto-braking.
The Capt.’s decision not to use auto braking was in fact the correct one as it was in accordance with the requirements of the auto ground spoiler system inoperative MEL procedure, which required that the auto brakes not be armed for landing in the case of auto ground spoiler system failure.
Without ground spoilers or auto braking, the only deceleration forces acting on the A/C after landing were provided by the thrust reversers. However, the time taken for thrust reverser activation and thrust reverser deployment meant that the peak reverse thrust did not occur until 24 seconds after touchdown. At that time, the A/C still had a ground speed of 137 knots.
The application of reverse thrust however, could have actually contributed to the difficulty in maintaining directional control during the landing. This was especially the case when the A/C longitudinal axis was not aligned with the direction of travel.
Since reverse thrust is a vector quantity, once the A/C longitudinal axis differs from the directional of travel (as in a skid or crab), a portion of the total reverse thrust retardation force will tend to drag the tail to the outboard side of the skid; that is to say the application of reverse thrust in this case will deepen or worsen the skid.
Incident Facts
Date of incident
May 8, 2009
Classification
Accident
Airline
Saudi Arabian Airlines
Departure
Jeddah, Saudi Arabia
Destination
Riyadh, Saudi Arabia
Aircraft Registration
HZ-APW
Aircraft Type
McDonnell Douglas MD-90
ICAO Type Designator
MD90
This article is published under license from Avherald.com. © of text by Avherald.com.
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