Nose gear door rigging and unsafe light after retraction test
A twin piston retractable-gear aeroplane is on jacks for a gear retraction check. Hydraulic pressure is 1 800 psi. The nose gear door closes but leaves a 2 mm gap at the forward edge, and the nose gear unsafe light flickers when the door is tapped. The nose downlock microswitch roller is worn 0.5 mm and the switch actuation gap measures 1.4 mm. The maintenance manual gives a door clearance of 1.5 to 3.0 mm and a switch actuation gap of 0.8 to 1.2 mm. Gear retraction time is 8 s, within the 6 to 10 s limit. The hydraulic reservoir is 0.3 L below the full mark, and the gear warning horn sounds at 15 in Hg manifold pressure with the gear up. The aircraft is due for release after a 100-hour inspection. The AME must decide what to adjust, what to test and whether the gear warning system is acceptable.
1. Evaluate the door clearance and microswitch measurements against the manual. What is the likely cause of the flickering unsafe light?
Read the worked answer
The 2 mm door gap is inside the 1.5 to 3.0 mm clearance range, so the door position alone does not explain the flicker. The microswitch actuation gap is 1.4 mm, above the 1.2 mm maximum, and the roller is worn by 0.5 mm. That combination points to lost motion in the switch mechanism: the door may be rigged acceptably, but the worn roller and excessive gap leave the switch on the edge of its travel. Tapping the door then changes the indication. I would replace the worn roller, adjust the switch to the specified gap, and recheck door rigging and the sequence valve. I would not bend the switch arm or enlarge the tolerance to make the light go out, because the unsafe light is the only indication of nose gear position.
2. What operational tests and hydraulic checks are required before release?
Read the worked answer
After adjustment, I would cycle the gear several times on jacks and check retraction and extension time, downlock engagement, door clearance and switch continuity at each position. I would check for hydraulic leaks and top up the reservoir to the specified level using the approved fluid, then recheck the level after cycling because the 0.3 L deficiency may indicate a leak or trapped air. I would also perform the emergency extension check and confirm that the unsafe light behaves correctly in transit, up and down. The gear warning horn must be checked with the gear up and the throttle retarded, and the test must be repeated for both engines if the system uses separate switches. A final inspection of the nose gear bay for chafing or interference is needed before release.
3. Is the gear warning horn at 15 in Hg manifold pressure acceptable?
Read the worked answer
I cannot accept 15 in Hg just because it sounds plausible. The maintenance manual or the aircraft flight manual gives the approved throttle position or manifold pressure range for the warning. If 15 in Hg is within that range, the horn is acceptable provided it cancels when the gear is extended and the throttle is advanced. If the range is different, the switch or linkage must be adjusted. I would also test the horn with the gear up and both throttles retarded, because a twin may use a combination of switches. The horn is a warning system, so an incorrect setting is an airworthiness issue even if the gear itself operates normally. The result of the test and the approved range should be recorded.