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Sitting on the runway about to takeoff, and the captain comes over the intercom saying that we need to return to the gate.

From the quick announcement, it sounded like he said they found a 'shear pin' at the gate, or with the tow-bar, and we needed to have maintenance inspect the front landing gear.

The aircraft is an A321.

I can't imagine the shear pin was a part of the physical landing gear, but was probably piece of the tow-bar itself? I found the manual for the a320's tow-bar which shows the shear pin in the diagram on PDF page #11, item #6, within the bar assembly itself.

Why would our aircraft need to return to the gate for inspection, wouldn't they need to inspect the tow-bar instead? That says with the tug, does it not?

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The Capt would have just repeated or paraphrased whatever Maintenance Control told him. If a push back is done with the nose wheel scissors connected, it is critical for the tug driver to respect the steering angle limits of the nosewheel to avoid bottoming the steering actuators and stressing the parts.

Although the shear pin protects (hopefully) from major damage it still puts significant loads on the scissors and nosewheel steering system, beyond what those components would normally see. If they find the towbar shear pin damaged or loose or sheared completely (although that should have been obvious while connected) after a push back, Maintenance will have to assume there was a steering angle exceedence event and there is likely to be a maintenance requirement to inspect the steering system, hence the return to gate.

My guess is that when the tug got back to the gate, someone noticed the pin was deformed, or some other abnormality, and they brought the airplane back for the inspection to play it safe.

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    $\begingroup$ Neccessary or not, kudos to the guy who caught the issue and got the airplane back to be checked. It's people like this who play a part in ensuring safety is never compromised. $\endgroup$ – Anilv May 21 '18 at 6:02
  • $\begingroup$ You can avoid the risk of steering angle exceedence by pushing back with the scissors disconnected (the scissors knuckle has quick-release T-handle plungers and the upper scissors link has a spring to hold it up), but now the crew has no way to confirm for themselves the plungers were re-engaged, and if not fully seated the scissors can come apart later, with potentially unpleasant consequences; hence pushing with scissors engaged, requiring extreme care by the tug driver and the shear pin in the bar just in case. $\endgroup$ – John K May 21 '18 at 12:08
  • $\begingroup$ I remember the Douglas DC-8 and 9s having the quick disconnect on the scissor (drag-link) but on most of the other types its a hollow nut and bolt, lock-wired.. not easily removed and connected. $\endgroup$ – Anilv May 22 '18 at 1:23
  • $\begingroup$ Depends I guess. All the RJs have the quick disconnect. $\endgroup$ – John K May 22 '18 at 3:27
  • $\begingroup$ Thanks JohnK. haven't had much experience withe these. Live and learn! $\endgroup$ – Anilv May 22 '18 at 6:59
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I’m going to give you a little bit of maintenance advice from a mechanic who has been dealing with stuff like this for 20 years:

Rule No. 1: Never, ever take the pilot’s words at face value. This is not a sleight against pilots but they just don’t speak the same language as we do. They are not taught to troubleshoot, investigate, or do anything remotely close to helping you fix the airplane. If they say “shear pin” it’s about as valuable as them saying “magical bolt.” I don’t mean that to sound nasty, but if you followed the flight crews’ recommendations on how to fix the problem, the plane would just sit forever. Their training and our training are simply not equal.

Rule 2: You are the Doctor, the pilot and his airplane are the patient. This falls somewhat in line with Rule #1. When a patient comes in and says something is wrong with his leg, that’s about all a doctor gets. That’s the level you should keep it at. Ask any doctor and they’ll almost completely agree that having patients diagnose themselves is NOT a good idea. Are they completely incapable? No. However, their depth of knowledge and access to information is simply nowhere near as great as ours is. Don’t let them steer you into the land of outliers.

Rule #3: Occam’s Razor: The simplest explanation is often the best explanation. That is... maybe a nose landing gear valve is stuck and the crew, knowing nothing about the world of aviation maintenance, declared it as a shear pin. The valve is most likely stuck due to mechanical failure or someone left the LOCKOUT pin installed while performing maintenance. Again, refer to rules 1 and 2.

Rule #4: A flight crew who cries wolf is actually right once in a while. This seems contradictory to rules 1 & 2 but it’s not. You see this sometimes with a flight crew member who consistently is incorrect about something. You see their ID number, remember it, and then think “Oh yeah... This is the guy who should be driving a bus, not an airplane.” As a result, you just sign it off. This is a fundamental flaw, like that of a doctor with a hypochondriac patient... you should always investigate on the basis of greatest caution.

Rule # 5: Don’t let anyone ever push you into a diagnosis or an ETIC. If the plane is down for 25 years and growing roots because of your estimated repair time, then so be it. By all means, do everything you can to get the plane up in a safe manner, but don’t cut corners to make a lead or manager happy. They can go to hell. Just investigate and do what’s necessary.

Rule # 6: If you don’t believe me about Rule #1 and #2, get into a conversation with a pilot about how flight crews enter their flight plan into the FMS. You’ll soon find out that you know nothing about it and they know everything about it. Then show them the system schematic of the FMS system and you’ll see their eyes get a befuddled look at what it all means while you stare confidently ahead. We just don’t speak the same language, PERIOD.

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    $\begingroup$ This seems more like an extended comment to me @Frank, as it doesn't actually answer the question. Granted, we pilots can be like computer users to tech support specialists, you always start from the beginning, but that doesn't mean you ignore everything that's said by pilots or cabin crew. $\endgroup$ – GdD May 21 '18 at 7:48
  • $\begingroup$ @GdD I did answer the question (refer to Rule #3). The declaration of a towbar shear pin when one clearly doesn’t exist is exactly what the OP is asking about. It might have been a bypass pin - which is a common thing - but it’s not a shear pin. Pilots really have no idea that what they say can be taken in many different ways because of the psychology of them “being in command of $\endgroup$ – Frank May 21 '18 at 7:53
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    $\begingroup$ your rules do not translate to an answer for me, Rule 3 does not stand out, if you're saying that the shear pin doesn't exist then I'd suggest making that clearer. $\endgroup$ – GdD May 21 '18 at 7:55

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