AAIB Bulletin: 1/2020 EI-DMG EW/G2019/06/19

of the safe or unsafe position of the landing gear. Three green landing gear down and locked lights, one for each landing gear, are located on the instrument panel and illuminate when the downlock switch on the respective landing gear indicates that it is fully down and locked. A single red landing gear in transit light illuminates when any, or all, of the landing gear are unlocked or in transit. The landing gear warning system sounds an intermittent audible warning if one or both power levers are retarded to flight idle and at least one landing gear is not down and locked. The warning horn may be silenced for this condition and will be reset when the power lever is advanced. The warning horn will also sound if the flaps are extended beyond the approach condition and at least one landing gear is not down and locked; the warning cannot be silenced in this condition. Electrical power is continually present at all the landing gear position indicator lights when the landing gear circuit breaker is closed. An individual ground circuit for each light will cause the light to illuminate when it senses an electrical ground. The nose landing gear downlock switch is a two-pole switch. One side of the switch provides the ground circuit for the nose landing gear down and locked light and the other side provides the ground circuit for the landing gear warning horn. Examination of the aircraft At the time of publication of this report, the maintenance agency had only conducted a preliminary examination of the aircraft’s nose landing gear system, with further, detailed, examination scheduled for a later date. Neither the cause of the nose landing gear’s failure to deploy, or the failure of the landing gear indication and warning system to alert the pilots to the unsafe condition had been determined, but the maintenance agency suspected it was related to a failure of the downlock switch on the nose landing gear actuator. Information from aircraft manufacturer The aircraft manufacturer identified several possible conditions which could explain why the nose landing gear down and locked light was illuminated despite the nose landing gear not being fully extended. The manufacturer confirmed that if the nose landing gear down and locked light senses an electrical ground, it will illuminate regardless of the nose gear downlock switch position (Figure 3). It was therefore possible that a faulty downlock switch could provide a continual ground to the down and locked light. Other potential sources of electrical ground could include an electrical short in the wire between the down and locked light and the downlock switch; inadvertent grounding of post 9 on terminal board 1; incorrect wiring on terminal board 1; or a defect within, or shorted wire from, the annunciator logic module to post 9 on terminal board 57 ©Crown copyright 2020 All times are UTC AAIB Bulletin: 1/2020 EI-DMG EW/G2019/06/19 Figure 3 Landing gear and flap warning system from Cessna 441 Wiring Diagram Manual Landing gear warning horn Annunciator logic module Nose landing gear downlock switch Post 9 on terminal board 1 Nose landing gear down and locked light 58 ©Crown copyright 2020 All times are UTC AAIB Bulletin: 1/2020 EI-DMG EW/G2019/06/19 The manufacturer also indicated that a faulty downlock switch could provide a possible explanation for the failure of the nose landing gear to fully extend. If the nose landing gear downlock switch had failed in the down and locked position, when the landing gear selector switch was selected to down, an immediate path to ground would exist for the nose landing gear down and locked lights and also for the hydraulic bypass valve. This would cause the bypass valve to open and allow hydraulic fluid to be routed back to the hydraulic reservoir, returning the system to its ‘at rest’ position. The main landing gear of the Cessna 441 are assisted by airflow to reach the down and locked position. The nose landing gear is extended against the airflow and may take longer to extend. Therefore, it may be possible for the main landing gear to be driven by hydraulic power to the down and locked position but, upon receipt of an erroneous ground signal from the downlock switch, for hydraulic pressure to be prematurely removed before full extension of the nose landing gear is achieved. A failure of the downclock switch in this manner would also remove the path to ground for the landing gear warning horn, preventing its operation. Landing gear downlock switches Cessna Service Letter SNL89-3 ‘Main and nose landing gear actuator switch sealing’ dated 17 March 1989 and Cessna Service Bulletin CBQ90-1 ‘Landing gear actuators microswitch inspection and sealing procedures’ dated 23 March 1990 indicate that the downlock switch on the Cessna 441 main and nose landing gear can be susceptible to ingress of moisture or other contaminants. This can contribute to switch malfunctions and possible incorrect landing gear indications. EI-DMG was among the aircraft serial numbers identified as requiring the downlock switches to be inspected and sealed. Discussion The commander reported that upon selecting the landing gear down during the approach to Prestwick all three landing gear down and locked lights were illuminated green. There was no activation of the landing gear warning horn and the landing gear in transit light was not illuminated. Neither had there been any anomalies noted with the landing gear before or during the flight. The pilots were not therefore aware until landing that anything was amiss with the nose landing gear. Although acting in a mainly observational capacity, the senior pilot instinctively acted to assist the commander when it became apparent that the nose landing gear was not down and locked. Both pilots reacted promptly to keep the aircraft nose off the runway and to activate the landing gear emergency blow-down system. Their recognition of the need to extend the emergency blow-down system during the landing roll ensured a successful outcome and minimised damage to the aircraft. The aircraft manufacturer described several possibilities which could create an erroneous ground signal at the nose landing gear down and locked light causing it to illuminate even though the nose landing gear was not fully extended. It was determined that one of these, a nose landing gear downlock switch failed in the down and locked position, could also account for improper extension of the nose landing gear and absence of the landing gear warning horn activation. 59 ©Crown copyright 2020 All times are UTC AAIB Bulletin: 1/2020 EI-DMG EW/G2019/06/19 While at the time of publication it had not been determined if the nose landing gear downlock switch on EI-DMG had failed, the Cessna 441 landing gear downlock switches are known to be susceptible to moisture and/or other contaminant ingress. Although downlock switches are required to be sealed, a degraded or damaged seal could provide a path for moisture to enter the switch. Conclusion As the aircraft had not been examined by the maintenance agency at the time of publication, it was not possible to determine why the nose landing gear failed to deploy correctly, nor why the pilots were not alerted to this unsafe condition by the landing gear indications and warnings. However, a faulty nose landing gear downlock switch was suspected as a potential cause.