Does this thing know where we are?
Modern airplanes are incredibly reliable. In the twenty-five plus years that I have been flying jets, there are certain issues that tend to come up as problematic more frequently than others. On the Bombardier CRJ series, some of the duct fans seem to fail pretty regularly, and the air conditioning packs were known to be temperamental. For a while, the cargo smoke detector was a major cause of problems, as it seemed that anything would set it off, including the radios that ramp agents had on them while loading and unloading the cargo.
The Boeing 737 I fly now is known for its dispatch reliability, and in the 10 years that I have flown it, I can count on just a couple of fingers the number of mechanical cancellations it has caused me to have. It seems with this airplane that it isn’t so much a matter of any systemic items that come up, but little gremlins that tend to be unique to one airplane. I’ve seen logs of an airplane having a ‘known issue’ that has been difficult for the mechanics to troubleshoot. On one airplane, for instance, there was a known problem with the cockpit window heat, but little success in finding the culprit. I’ve no doubt that the airplane finally got sent to a hangar in a maintenance base to get that one figured out. Another one that I flew several times in short order had a weird issue with the intercom on the captain’s side, and then only with one brand of headset. The mechanics thought I was crazy until they experienced it themselves.
But there are some issues that just never seem to come up. I’ve had avionics issues, but never have I lost a flight director. Another incredibly reliable system is the landing gear. Most landing gear require some form of hydraulic pressure to either raise it, lower it, or hold it up. On jets, they tend to be held in the retracted position by some kind of an uplock, and when the gear handle is put down, the weight of the gear tends to bring it down, with hydraulic pressure only helping out.
Once on an CRJ, I had a nosewheel get hung up briefly during the extension. The first officer that I was flying with was fairly new, so he wasn’t yet intimately familiar with the sounds that were typical of normal systems operation. However, I had thousands of hours in the airplane, so when I heard the nose wheel hit the doors without opening said doors, I noticed it right away. Sure enough, we started getting “gear disagree” alerts. Fortunately, the doors gave way, and the nosewheel came down and locked itself.
On the 737 recently, we had a similar issue, but it occurred during retraction. The 737 has two sets of gear indicators, one on the front panel, and one on the overhead panel. Boeing states that if either set of indicator lights is green, the gear is considered down and locked. In our case, we got a mix of lights on takeoff. It was again for the nosewheel. Even though we had one set of green lights, it was clear that the nose gear was likely still up. There was none of the noise associated with the nose wheel doors being open, and the power requirements for level flight indicated that we were ‘clean.’
Because we needed to burn off fuel to get to our max landing weight, we agreed to honor the company request to fly to a nearby hub, where maintenance and passenger resources would be better. When we extended the gear, it was clear that the nosewheel had been at least mostly retracted, if not all the way up. All three sets of wheels came down, and we landed normally.
It turned out to be a proximity sensor. Transport category airplanes all have one or more of these sensors, and in layman terms, it is a sensor that helps the airplane determine if it is on the ground or in flight. This is necessary for the computer logic for multiple systems that operate in different modes based on phase of flight, to include being airborne or not. Pressurization, flight control logic, fire detection, and even the emergency exits are examples of systems that take their cues from the proximity sensor(s).
In our case, once we got on the ground, the gear lights and proximity sensor switch began to act even more strangely, and fortunately, the mechanics were able to see what we were seeing while replicating the problem. To put it simply, the airplane didn’t always know if it was airborne or on the ground.
The fix wound up taking the better part of a day, and required several part replacements, but it was fixed and returned to service. The oddity of a gear issue was reinforced by our interaction with the mechanics, as they were just as flummoxed as we were. Gear issues do happen, but they are relatively rare, and it can take some head scratching and teamwork to figure them out. Systems knowledge and familiarity can go a long way, and in our case, the teamwork between us as pilots and the mechanics was very complimentary.
An old adage one of my instructors used to quote all the time was echoing in my head: know thy airplane and know it well. It was good advice then, and it’s good advice now.
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