That dang gear thing
Ya see folks, in every day ops, there are a myriad of "pins" inserted into the downlock of the 747 nose gear. It's a throwback to the 707 days when the nose gear could be pulled up during tow.
These are not meant to keep the gear from folding up if you power the system, pull the trigger override and put the gear handle up. They're there to keep the TUG from pulling the gear up. A belt and suspenders sort of thing. NO pin should REALLY be required for the downlock for routine ops (towing/pushback), but we are a superstitious lot and nobody really wants to prove that the pins aren't needed for a tow.
Having said that, there are a number of maintenance checks that require the gear be pinned with weight on wheels and the gear handle being placed in the up position and the system pressurized.
These are not infrequent in the hangar, but rare on the line. In any event, the rarity of this procedure on the line negates any excuse for not actually climbing up the strut and inserting the actual mark 1 mod 0 manufacturers gear pin.
Oh yeah, if I were doing a leak check, I would have opened the gear doors manually and locked them there via the ground handle. Then you could cycle the gear handle all day long and the doors would stay open. Both sides of the system would be powered, but the doors would stay open. In any event, this inadvertant retraction is NOT an uncommon event (unfortunately).
If memory serves me, a common reason for a nose drop is an AVO changing a #2 INS control head. In order to extract the box, the gear handle has to be up. AVO puts gear handle up, replaces the box, then goes for tea or coffee or some other distraction. Somebody powers the system and you get that sinking feeling.
Getting back to the pin getting pulled out by the doors, that's sort of unlikely. IF you are using the routine fishpole pin, I believe you have to tie the bar to the strut to prevent the thing from swinging forward and getting stuck between the doors when you put the handle back up and close the doors. Otherwise, you will be changing a pair of gear doors. I could be wrong on that point, it HAS been a while since I pinned a 747, but I am fairly certain.
Finally, you are not really locking the gear, you're locking the down lock. It's an overcenter design, so theoretically, looking at it funny should stop it from going over center. Could be wrong there as well, but.....
It IS possible that if the lock on the gear pin was defective, the vibration caused by moving the gear handle up and down (the airplane shakes and rattles when you do this procedure) COULD have caused the pin to fall out, but if there was a person standing there, ostensibly on the headset and looking for a leak, he or she would have seen the pin fall out if it didn't hit them on the head.
A rambling reply, but someone is destined for a management position. Of this, I am sure.
PB