I am inspired by Llama Llogophile’s comment in another thread (http://boards.straightdope.com/sdmb/showpost.php?p=19390093&postcount=4) to ask about a recent flight. I was on what I believe was a DeHavilland DHC-8 400 operated by a small European carrier. We took off on time but perhaps ten minutes into our scheduled two-hour flight over water, the pilot announced that we were returning to the airport because of a door sensor problem. This counts as the shortest commercial flight I’ve ever been on.
We disembarked when we landed but the plane was fixed in less than 15 minutes. We climbed right back on board and got to our destination with no apparent difficulty.
Is there a type of door sensor whose failure would only be apparent after the plane took off or if the cabin was pressurized? Could it really be fixed that quickly? Would the pilots have just lied about why we returned to the airport? What are the chances we faced any actual danger? I certainly didn’t feel endangered but I’m pretty comfortable on developed world carriers. Maybe I should have been nervous after all.
Door sensors are mechanical devices and NOTORIOUS for needing maintenance/adjustment over a period of time. We are talking burglar alarm door sensors, washing machine door sensors, and yes, aircraft door sensors.
I would imagine the most probable door sensor problem would be with a cargo door. Those baggage handlers tend to throw things around and perhaps slam doors closed. A sensor could be knocked out of adjustment.
With that said, did you see anyone going under the plane to work on something? Or did they just open/close the main entry door?
There are also many other doors - wheel well doors, fuel filling doors, maintenance access hatches, etc.
After we landed, the pilot announced that passengers had to walk back to the terminal but that we would depart again in fifteen minutes. About five minutes after I got to the terminal, they announced that the flight was re-boarding. When the pilot announced that our delay would be so short, I assumed we were taking another plane. My wife assumed that he was lying to us. We were both wrong. Instead, we got on a bus and they drove us right back to the original plane within minutes. My seat was still warm. There were about five or six ground personnel lingering around the plane when we arrived so I assume they did something to the plane when I wasn’t paying attention. I’m curious what that was.
Might have been a door seal. I don’t know the DHC systems, but some planes have a flexible seal around the cabin door that pressurizes to close a gap. Could be that it didn’t work, so you turned around and landed. Not sure how quickly that could be fixed though.
It seems to me that it’s a good idea to have complete transparency in releasing every airline’s overall maintenance policies and statistics to the general public, in a standardized format that allows us to make like-for-like comparison between airlines. As a passenger, I’d like to know an airline’s overall history of reprimands and violations; I’d like to know if their maintenance policies exceed mandatory minimum standards, and if they have a record of sticking to those policies consistently, without putting pressure on crew to fly on schedule in marginal circumstances.
However, I don’t think that it makes sense to inform passengers of the technical details of minor maintenance issues that may affect a particular flight. Most passengers cannot possibly understand the significance or safety implications of a specific issue.
In other words: I want to know if an airline has good safety policies and a good record of adhering to those policies even when it costs them money. I will choose my airline accordingly, perhaps paying more for my ticket than the cheapest available. But having chosen, I’m not qualified to micromanage or second guess what they are doing, I have to trust them to get on with it.
I was delayed last year due to a sensor fault. When they finally let us board, there was a mechanic on a step ladder hitting part of the engine with a hammer. Presumably to seat the new sensor.
Several passengers turned around and headed back to the terminal with very scared looks upon their faces. Maybe they should have delayed the boarding just a little bit longer?
Actually one of the best mechanics I have ever known was an aircraft mechanic. He would do things right and by the book! (That is everything he worked on including cars - one of the best car mechanics around.)
In the past, flights have suffered explosive decompression at cruising altitude due to faulty door latches, particularly, as noted earlier, on cargo doors.
I’m not the least bit surprised that they headed back to safe land to confirm everything’s properly latched and secure, rather than hoping it’s just a funky sensor.
I used to fly the Dash 8. I don’t have details for the 400, but the abnormal checklist for the 300 model says:
There is not, as far as I know, a warning to say the sensor itself has failed, so I think when the pilots said it was a door sensor, what they meant was the door sensor indicated a problem (i.e., a door is not closed properly.) The checklist assumes the worst case scenario, a door is actually not closed properly, and directs the pilots to:
[ol]
[li]Turn the seat-belt signs on so the passengers remain seated.[/li][li]To depressurise the cabin so as to reduce the internal pressure pushing out on the door. To land at the nearest suitable because if the door does actually open fully you don’t want to be flying around any more than you have to.[/li][li]And finally to complete the unpressurised flight checklist because you’ve just depressurised the cabin.[/li][/ol]
It is most likely though that there is a fault with the sensor. So once on the ground the applicable door’s function would be checked and if found to be serviceable, the sensor might be replaced or adjusted. This doesn’t necessarily take a lot of time, particularly if engineering have been informed and have spares on hand.
To answer your questions and because I feel like making another list ;):
[ul]
[li]Yes there are sensors for the doors and a misaligned one is more likely to detect an open door once the cabin is pressurised.[/li][li]Yes it could really be fixed that quickly.[/li][li]It is unlikely the pilots would have lied to you. If they don’t want to give details they’d be more likely say something like they “have a technical issue requiring a return to base” rather than making something up. If they give details, those details are most likely accurate though they may be put in layman’s terms that can make it difficult for another pilot to decipher exactly what the issue was when told second hand by a passenger.[/li][li]It was a precaution, you should not have felt nervous.[/li][/ul]
Thanks Richard Pearse! I appreciate the informed response and I am glad to know that I didn’t really have any reason to worry. It seems like the protocol you described was the one the pilot followed. It also makes more sense that the pilot wouldn’t bother to lie to the passengers when he could instead elect to tell them nothing.
I sell Aircraft door sensors and it should be noted that they actually measure the gap between the sensor and the target and so the door could be closed and latched, but if the compression on the seal isn’t as expected, the sensor will register a fault.
I test and rig the door sensors on new 737’s in the factory. These jobs are done after the door itself is rigged. On the 737 and other Boeing aircraft, the door sensor has nothing to do with the door compression seals, a bad seal will not cause a failure. The sensors on most of the doors are simple proximity sensors. The targets are adjusted to meet engineering specs then safety wired. Another job tests the system electrically, open a door, the door warning light and master caution illuminates, close the door, they both extinguish. All are controlled through a box in the nose of the plane, an item called a proximity sensor electronic unit. A pressurization fault won’t be caught for about 5 minutes after the plane achieves air mode, this happens when the landing gear fully extend when the plane lifts off the runway. This is also tested in the factory, we can simulate air mode through the PSEU then induce faults in the sytem. This test is run during what we call high blow, the fuselage is pressurized and checked for leaks.