Eastern Airways E170 at Birmingham on Jun 7th 2021, trim, autopilot and steering failure
Last Update: October 13, 2022 / 14:16:19 GMT/Zulu time
On Oct 13th 2022 the AAIB released their bulletin concluding the probable causes of the serious incident were:
The failure of the pitch trim system probably occurred because of jamming of the horizontal stabiliser as a result of freezing of water ingression in the ball nut due to condensation. The investigation found that the operator had lubricated the horizontal stabiliser at the intervals specified by the manufacturer to prevent such an occurrence.
Jamming of the horizontal stabiliser resulted in the loss of the pitch trim functionality. Consequently, the PF experienced greater than normal nose-down pitch forces on the approach. The PF delegated the monitoring of the throttles, which were still in autothrottle, to the PM to allow him to use both hands on the yoke. The PF flew a stable approach and made a safe landing. However, on landing, the steer fail caution illuminated, as a result of an unrelated sensor failure. The commander took control and brought the aircraft to a safe stop and taxied the aircraft to stand using differential braking.
The AAIB analysed:
The fixed position of the horizontal stabiliser was probably caused by the jamming of the ball nut due to the freezing of water ingress, itself probably the result of condensation. Although examination of the HS-ACE only identified a single channel failure, this condition alone should not prevent the surface movement as the channels are automatically switched after the failure of one channel. (The system is designed so that only one HS-ACE channel is active at a time and is able to command the system). Since the HSTAB was fixed, the active channel was not able to command any movement. After switching, the standby channel became active but could not command movement as well for the same reason. This condition resulted in the loss of pitch trim functionality and illumination of the PITCH TRIM FAIL caution.
The disengagement of the AP by the PF, even though this is not required by the relevant procedure, addressed the hazard of a more potentially serious trim condition of a pitch trim runaway. AP trim functionality, indicated by the illumination of the ap fail caution, would not have been available owing to the pitch trim failure.
Failure probably resulted in the HSTAB being stuck at -2.4°, (and that the two occasions, where -2.4° was recorded, were the result of the crew completing the actions which required the de-selection and re-selection of the pitch trim cut-out switches as part of the pitch trim failure procedure.) The consequence of the stabiliser being stuck was that the PF experienced heavier nose-down forces in pitch than would be normal, requiring stronger than normal pilot inputs to maintain the correct pitch attitude for the approach. The decision by the pilots that the PM would monitor the throttles, which were in autothrottle, to enable the PF to place both hands on the yoke to control pitch attitude, gave the PF the control to make the required pitch changes. The approach subsequently flown was stable and resulted in a safe landing.
The investigation found that the operator had lubricated the HSTAB in accordance with the recommended intervals, which were intended to prevent such an occurrence, as specified by the manufacturer.
The steer fail caution was triggered by a sensor failure. This resulted in the loss of steering through the pedals (and the handwheel) which required the PF to steer the aircraft in free wheel mode using differential braking.
The pitch trim fail and steer fail cautions were unrelated. The nature of the failures, affecting control both in the air and the ground, together with the flt ctrl no dispatch CAS message required further action by the operator before the aircraft could be declared serviceable, necessitating an entry in the aircraft technical log by the commander.
This article is published under license from Avherald.com. © of text by Avherald.com.
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