Gear Disagree questions

[torrence]

I've been working with the Alternate Gear Extension procedures in PSX.  When I activate all the Gear Disagree options to Severe on the Malfunctions page the gear can be lowered as expected by depressurizing Sys1 hydraulics for the Nose and Body gear and Sys4 hydraulics for the Wing gear and using the Alternate Gear Extension switches, but I've got two questions:

1. Timing - one FCOM I have has considerable discussion of how long the alternate extension procedure might take, saying in one place that it might take up to 15 min and discussing in another note cycling the control wheel to speed up depressurization of the hydraulic lines.  In PSX the extension seems pretty fast - I got about 30 secs after I turned off the hydraulics and punched the Alternate extension switches but the subsequent flap extension was slowed of course.  What factors affect the timing in this type of malfunction?

2. If all the gear are not extended (the case I've been using), would you in real life work to get all the gear out no matter what, or would there be factors like fuel, landing conditions, winds etc. that might lead you to just get the Sys1 Nose and Body gear down as priority?  There's a lot of  phrases like "... land with available gear ..." in the checklists I've seen.

Cheers
Torrence

[localiser]

Hi Torrence,

To answer your second question, the Boeing FCTM says you should land with all available gear and that a partial gear up, or gear up landing is preferable to running out of fuel troubleshooting or fretting about a gear problem. Airbus on the other hand only say that it is always better to land with any available gear rather than carry out a landing without any gear.

Fuel burn considerations to one side, taken on its own this is a high severity but actually low time pressure scenario. That is to say, the landing manoeuvre with abnormal gear is risky but as long as you have plenty of fuel, you could fly a long time without any great urgency to land with a gear problem - unlike with an engine fire. VS043 was airborne for around 4 hours.

However, as you point out correctly, there are many operational things to consider in real life decision making and fuel is one of them. Fuel burn penalties come into play when you have problems with extended gear or flaps and when towards the end of a flight this increases time pressure. If a problem occurs at the beginning of the flight and there is time to jettison fuel, and perhaps hold for a while after that as well, that time pressure is released somewhat.

The way the manuals are written, I would take the phrase "land with all available gear" at face value. The crew of VS043 had a scenario not covered in the non-normal checklist. The QRH checklist for alternate gear extension did not offer an option for the case where all the gear are not down after extension (see the report's findings), so they had to find a way to deal with this inconsistency.

Similarly, the open-seeming phrase "consider an immediate landing" in the smoke drill is not a very pleasant one when you think about it, and essentially intends an off airport landing in a worst case situation.

The links below to G-VROM VS043 may be of interest to you. The "History of the Flight" section in the report gives you a good idea of the technical and operational considerations the crew had. Operational in this sense means things like ATC, cabin crew, passengers, maintrol/engineers, using any extra crew members, where the company wants you to land, landing performance, (cross)wind and weather, daylight and so on.

https://youtu.be/ZqDP-FMgTy8

https://www.gov.uk/aaib-reports/aaib-investigation-to-boeing-747-443-g-vrom

[Hardy Heinlin]

Hi,

re VS043: In the video one can see that the right outboard elevator is inop as hyd sys 4 is depressurized. During rollout when pushing the tail down, I think this elevator asymmetry helps a tiny little bit to bank the aircraft more towards the left good wing gear rather than towards the retracted right wing gear where the engine pods are already very close to the ground. If the right wing gear had been retracted it would have been even more difficult, perhaps. A tiny little bit. I think.


Regards,

|-|ardy

[United744]

I would guess you try your best to get all the gear down, and after some attempts you land with what you have when you're happy.

I'd guess being as light as possible is always one goal, as it reduces the landing speed, reducing the braking effort required, etc.. so lighter is better in that respect.

You could try pitching up/down to help get the gear extended, too, or rocking side to side, or even side-slipping, to try and get the air to grab the gear and help extend it. Depends on what you're trying to do.