Flybe DH8D near Exeter on Nov 15th 2018, altitude disagree
Last Update: October 31, 2019 / 18:41:42 GMT/Zulu time
The AAIB reported a post flight inspection found contamination in the static ports. The occurrence was rated a serious incident and is being investigated.
On Oct 31st 2019 the AAIB released their final bulletin concluding the probable causes of the serious incident were:
Following scheduled maintenance of the incident aircraft, a small quantity of a siliconebased grease was blocking three of the four static pressure holes of the left primary pitot static probe. The inadvertent blockage of the static pressure holes resulted in an altitude mismatch of 140 ft between the commander’s and co-pilot’s altimeter. This may have been caused by using a non-approved grease to aid sealing the test adaptor to the pitot static probe, a task which can sometimes be problematic. The kit manufacturer’s recommended lubricant is sometimes missing from the kits and the AMM and the kits instructions do not include any details on installation or sealing.
The AAIB described the sequence of events:
Prior to the incident flight, the aircraft had been undergoing a standard maintenance check at the operator’s maintenance facility at Exeter Airport. This activity included cleaning and leak checks of the pitot static system. The aircraft was released for service on the morning of 15 November 2018.
The first flight, on that morning, was planned from Exeter to Charles de Gaulle Airport, Paris. The weather conditions were benign with a temperature of approximately 9°C. The commander completed a pre-flight walkaround of the aircraft and did not observe anything unusual. The aircraft pushed back from the stand at 0706 hrs. The pushback, engine start, and taxi were all normal. The pitot heat was selected on as the aircraft entered the runway in accordance with the operator’s standard procedures. The takeoff roll was uneventful and the 80 kt airspeed cross check did not reveal any discrepancy.
At approximately 500 ft aal, an alt mismatch message briefly appeared on the primary flight display. The flight crew reduced the rate of climb whilst they discussed the message. The mismatch message reappeared intermittently throughout the climb. The aircraft levelled off at FL190, where the flight crew recall the commander’s altimeter showed 18,860 ft, the co-pilot’s altimeter showed 19,000 ft and the standby altimeter showed 18,920 ft. The airspeed was consistent with the aircraft’s pitch and power setting but, the two primary airspeed indications showed a 3 to 4 kt difference.
The alt mismatch message continued to appear intermittently at FL190. The flight crew actioned the appropriate QRH drill and decided, in consultation with the operator, to return to Exeter Airport. The subsequent descent, approach and landing were uneventful.
The AAIB analysed:
During the incident flight the altitude difference between the commander’s and co-pilot’s instruments was 140 ft which equates to a pressure differential of 2.8 mb under ISA conditions. There was good evidence from the post-flight troubleshooting that three of the four holes on the left primary pitot static probe were blocked; this would have affected the pressure balancing between the left and right side of the aircraft. The S1 static pressure system had one hole open in the left probe and two holes on the right probe which resulted in the higher pressure (lower altitude) whereas the S2 system had only two holes open in the right probe and gave a lower system pressure (higher altitude).
It is possible that the difference of 2.8 mb between these two dissimilar system configurations could have been caused by a slight sideslip to the right which would increase the pressure on the right side of the aircraft and would not have been averaged due to the blockage of the left side S2 holes. Although on this occasion the altitude and airspeed errors were small and resulted in a successful return to the departure airport, a blocked pitot static system has the potential to cause a large error in altitude and airspeed information displayed to the pilots. Unreliable primary flight data has previously been a contributory factor in several accidents and serious incidents.
Work as done - blocked static holes ‘Work as done’ according to the Shorrock concept is the actual activity taken to complete the task and may occur in an environment that is subject to a variety of constraints, challenges and demands that are not ‘imagined’ or ‘disclosed’. The work done may be the product of adaptations to overcome these which, although intended to achieve the objective, may result in unintended consequences.
From interviews with the avionics technicians it is known that it can be difficult to achieve an effective seal between the test adaptors and the probe, and they indicated that several different methods have been employed to achieve a seal. Analysis of residue found on and inside the pitot static probe, as well as on the seals on the probe adaptor, strongly suggests that substances other than the recommended lubricant had been used during maintenance activity. There was also some evidence of damage to the knurling on the probe adaptor, and it is probable that hand tools had been used to tighten the locking sleeve, despite the air data accessory kit manufacturer stating that hand tight is sufficient.
This article is published under license from Avherald.com. © of text by Avherald.com.
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