We came into work today and at the rolecall me and another avionics colleague were requested to go to the line (The part of the airplane where the passengers board).

The flightcrew had noticed that during the flight the right main landing gear red warning light had come on, indicating that the right hand main landing gear is not up and locked. The uplock switch sends a signal to the PSEU (proximity switch electronic unit) that the position of the gear lever (in up ofcourse) and the up and locked switch of the right main landing gear disagreed. The flightcrew then has to assume that the main landing gear is not up and locked, when they then select the gear lever to down, the PSEU will no longer have a disagree between the switch and the position of the gear lever and the red R/H gear light will go out. The gear will go down and the down switch and a lock switch will indicate that the gear is down and locked properly. Because there is only one uplock switch per main landing gear this had to be repaired.

So we grabbed some special tools that we would need, jumped into a car and drove to the line maintenance station.
At the line we received some basic tools, a pat on the back of two avionic's colleagues that were surfing the web and drawing mustashes on pictures of other colleagues. They wish us good luck with the job and off we went.
Now this is a relatively easy job but like anything in aircraft maintenance, things can turn bad at even the smallest job's.

We arrive at the aircraft and look at the switch to check for any obvious damage it might have.
this is the switch
This is a proximity type switch, this means that there are three wires rather then the two that a reedswitch has. A reedswitch is a switch that reacts to a magnet being brought close enough to the switch to switch over the internal contact. A proximity switch creates a field itself and measures wether a target is in close proximity or not. The target of a proximity switch is therefore just metal and not a magnet.

We don't see any obvious damage to the switch or it's wiring so we agree to replace the switch. I start by removing the first bolt that holds the switch.
The bolt and the nut are easily accessable but the spacer in between is very hard to reach and the only way to remove it is to catch it when it falls.
The spacer falls right into my hand and we look at each other knowing that if we lose the spacer we have a serious problem. The proximity switch senses the target near of far but if the space between the switch and the target is changed we need to put the aircraft on jacks and do a gear retraction to find out if the switch is correctly spaced.
If we put the switch at the same distance, a gear retraction is not necessary and our deadline is now an hour and a half away.
bolt and spacer

I remove the other bolt and again, the spacer falls right into my hand. The switch is now hanging loose.
switch hanging loose

In the meantime my colleague has cut the tie laces (the small ropes that hold the wire loom together) and loosened the p-clips to get access to the wire splices where we are going to cut the wires.
My colleague then cut's the wires at the old splices.
wires cut

We have no tie lacing rope on us wich we normally connect to the old wiring to to pull through a conduit so that the new wires can be pulled back through the conduit by pulling on this tie lacing rope.
So we decide to just pull the wires out and hope that we can feed the new wires back through the conduit.
switch removed

This is the old switch that we just removed. We receive a call from the line maintenance station to verify if we can manage the job on time. We inform them that the departure time should be fine.
the old proximity switch

We try to feed the new switch's wiring back through the conduit but it just doesn't want to go in, we have to pull the wiring through after all. We take one of the old wires from the switch and push it through the conduit from the other side down to where the switch should come.
feeding a single wire

Time is now running out so we quickly attack the new wiring to the old wire and pull the whole lot through the conduit.
pulling the new switch through

There, the new switch is in place, we need to splice the wires together now and connect it all back up again. By this time fuelling department arrives to start fuelling the aircraft up for it's flight.
in position

The new wiring needs to be stripped and crimped into the splices. At this time the baggage handlers come up to the aircraft to load the bag's on board for the flight.
the new wiring

My colleague takes care of that.
splicing the wiring

The watertight seals are pulled over the splices and we heat shrink them to make them watertight. The captain comes to confirm if the departure time is still valid. We inform him that the job can be completed in time.
sleeves

My colleague works the wires back in the loom and rushes to the toolstore to get some tie lacing rope to complete the wire loom while I start re-connecting the switch.

I remove the gearpins from the landing gear and start the test while my colleague is tie lacing wire loom together.
wire loom


And that's the uplock switch replaced.
Header image
It was good line maintenance weather that day anyway.