Landing gear problems - BA295

[vito]

Just found this:

https://twitter.com/hashtag/BA295?src=hash

interesting details about disembarking

best regards

vito

[G-CIVA]

Just like the Virgin crew at LGW last year BZ to the crew.

Av Herald states that both only both Body Gear extended on finals after a long range return south of Keflalik enroute to SFO.

http://avherald.com/h?article=4932a82e&opt=0

The BA Source indicates that the ac involved G-CIVX had been under MX at LHR since Jan 25 2015.

http://www.thebasource.com/british-airways-b747-400-g-civx-ba295-heathrow-technical-return/

The BA Source goes on to say that the PAX were able to subsequently resume their journey to ORD on G-CIVI, something to be said for BA holding on to all of those paid for 744s .... so much extra capacity when needed.

[Jeroen Hoppenbrouwers]

Gear lever stuck up due to maintenance goof. Could not lower gear any more, pressure keeps it up, alternate extension won't work. Had to depressurize hyd #1 to get the alternate nose and body gears come out and keep them down; did not depressurize #4 to retain braking. Long landing due to half braking, pax had to disembark aft first to not sit the bird on her tail.

http://avherald.com/h?article=4932a82e&opt=0



Hoppie

[Hardy Heinlin]

Why the balance problem? The body gear is closer to the tail than the wing gear. Is it because the fuselage gets a bit closer to the ground, especially the aft part of the fuselage? I guess if only two instead of four gear are holding the center of gravity up, the fuselage sinks a bit below the normal height; and since the nose wheel has not much weight to hold up, the nose remains at the normal height, and the center goes down, and the tail even more. That's it, isn't it?


!-|

[Phil Bunch]

Gear lever stuck up due to maintenance goof. Could not lower gear any more, pressure keeps it up, alternate extension won't work. Had to depressurize hyd #1 to get the alternate nose and body gears come out and keep them down; did not depressurize #4 to retain braking. Long landing due to half braking, pax had to disembark aft first to not sit the bird on her tail.

http://avherald.com/h?article=4932a82e&opt=0



Hoppie

This landing strategy with compromised equipment sounds like good, highly skilled crisis management to this non-pilot.

Is this specific set of hardware problems and its solution explicitly taught to 747 pilots, or did this crew more likely use creative thinking and problem solving on an ad hoc basis?  I assume that they had time to work through alternatives (if any), consult with maintenance by radio, etc.

[Britjet]

Hardy,

I have no inside information on this so don't know what was discussed with company etc, but it is possible that they got a bit confused. The tail-tipping should only be an issue if both body gear are not down, as you surmise ( from the QRH)

Peter.

[Sylle]

Hi Phil,

This very problem is mentioned in the Boeing QRH under 'Gear Lever Jammed In Up Position'.
I suppose this has been the main guidance to come to the decisions the crew took that day.

Regards,
Sylvain

[Sylle]

Hi guys,

Need the help and technical insight of other members here...

I recreated the above issue with PSX by leaving the landing gear lever in the UP position and deselecting the F/O silent tasks to make sure it would stay in that position.

Everything worked out great until touchdown in EGLL.
It seems that in PSX the air/ground sensing system still thinks we are flying after touchdown.
Nothing in the checklist seemed to have drawn my attention to the fact that this could happen.

Side effects were:
- Autobrakes not engaging and not disarming when manual braking was used.
- Unable to pull reversers
- Unable to pull speedbrake lever beyond the flight detent (not armed in accordance with checklist)
- Unable to start the APU. Had to connect EXT PWR to shut down the engines and keep electrical power.
- Unable to pressurize hydraulic system 4 in AUX.

Did I overlook something??
Any idea why the systems are not transitioning to ground mode??

Regards,
Sylvain

[Hardy Heinlin]

Hi Sylvain,

one possibility to get the on-ground mode is to have any left and any right main gear down.

The system considers a main gear down when it is down AND either of that gear related hydraulic systems is pressurized -- this info comes from the associated HYDIM, i.e. it must be operative.

I.e. if the gear is down but both related hydraulic systems are not pressurized, the system considers the info invalid, i.e. it thinks it's up.

Another possibility to get the on-ground mode is to have the nose gear down and either nose gear related hydraulic system is pressurized and the nose gear is on the ground.

Maybe these factors are related to your question?


Regards,

|-|ardy


In the above scenario, are both wing gear up? I just tried this in PSX and I got the on-ground mode. Why do you put the lever to the UP position?

[Sylle]

In the above scenario, are both wing gear up? I just tried this in PSX and I got the on-ground mode. Why do you put the lever to the UP position?

Hardy,

Both wing gear were up indeed.

I did put the landing gear lever in the UP position after takeoff and then left it there on purpose to simulate the exact problem the crew of BA925 experienced.
I then used the checklist in the QRH titled 'GEAR lever jammed in UP position' to extend the landing gear with the PSX gear lever still in UP (simulating that it was stuck in that position as happened in real life).

The QRH doesn't speak about any air/ground sensing issues and even lets you select an autobrake setting which is why I was surprised to see it did not activate upon landing nor did it disarm when braking manually.

The FCOM is much more brief again than the explanation you posted above.
FCOM mentions that main gear untilt would also signal the aircraft to be 'on ground'.
I can't find that condition/situation back in your explanation.

Can we further check if the fact that both body main gear were untilted on the ground would not be sufficient to trigger the on-ground mode?

Regards,
Sylvain

[Hardy Heinlin]

I'm not quite sure what you are trying ...

You want to have the nose and both body gear down, and both wing gear up. Right?

How do you want to keep the nose and both body gear down while you keep the gear lever UP?

Can we further check if the fact that both body main gear were untilted on the ground would not be sufficient to trigger the on-ground mode?

I've learned that the "untilted" status is only valid if the related hydraulics are pressurized. I assume this is sort of a plausibility check to avoid false "untilted" detections, e.g. due to being stuck in transit or something like that. But this hydraulics check is only performed if the respective HYDIM is powered. If it's not powered, it doesn't matter whether the hydraulics are pressurized. The "untilted" status is then accepted anyway. You may try to pull the respective HYDIM CB or activate a HYDIM failure on the Malfunctions > Hyd page.


|-|ardy

[Avi]

The Air / Ground system is so complex and have many relays.
First of all you need to understand there are 3 situations to the system:
1.   Aircraft on the ground (ALL landing bogies (5) are not tilted).
2.   Aircraft in the air (ALL landing bogies (5) are tilted).
3.   Aircraft not on ground and aircraft not in the air (some bogies are tilted and some are not – your situation).

To complex things there are 2 sets of Air / Ground systems: Primary and Alternate (each other uses its own sensors and power sources).

Now, different systems / sub systems which use signals from the A/G system have different requirements for their operation. One system may needs both systems to agree while other system may need "good" signal from only one A/G system.
One system may need "Aircraft on the ground" signal while other system may be happy with only "Aircraft not in flight" (and as Hardy said hydraulics is also counted for some systems).
There is no "solid" role here for the entire aircraft systems.

Cheers,

[Britjet]

We flew this exact same scenario in Gary's sim some time ago, using the QRH to the letter, and it worked as advertised.
Peter

[Sylle]

Thank you Avi for the details....
There must be something I'm overlooking here...
The QRH should have some info if this was to be expected.

I'm not quite sure what you are trying ...

Easy! I'm trying to replicate the issues the BA295 crew had on their flight of 30 JAN.
As described in the avherald topic in Hoppie's posts above, their gear lever jammed in the UP position due to faulty rigging after maintenance. To be able to drop the gear they couldn't simply use the alternate gear extension as the lever in UP position would keep all wheels nicely tucked inside.
PSX does not allow to fail/jam the gear lever in UP so I'm simulating this problem by simply leaving the lever in the UP position after takeoff and imagining it's stuck there...

You want to have the nose and both body gear down, and both wing gear up. Right?

How do you want to keep the nose and both body gear down while you keep the gear lever UP?

Exactly... that is what the QRH asks.
The QRH let's you depressurize hydraulic system 1 to be able to drop the nose and body gear with alternate gear extension when the gear lever is jammed in the UP position.

We flew this exact same scenario in Gary's sim some time ago, using the QRH to the letter, and it worked as advertised.
Peter

Same here... gear dropped, flaps extended slowly as mentioned in the QRH... except upon landing the air/ground sensing did not transition to ground causing all these issues described above.
You tell me you were able to use the reverse, speedbrake and autobrake correctly when you tried this scenario?

Thanks all for the feedback!
Regards,
Sylvain

[John H Watson]

Another possibility to get the on-ground mode is to have the nose gear down and either nose gear related hydraulic system is pressurized and the nose gear is on the ground.

Either nose gear related hydraulic system? There is only one hydraulic system for the nose gear (system #1).  Did you mean either gear-related system unpressurised?

I've learned that the "untilted" status is only valid if the related hydraulics are pressurized. I assume this is sort of a plausibility check to avoid false "untilted" detections, e.g. due to being stuck in transit or something like that.

Another possibility is that when the gear is unpressurised, the truck (bogey) tilt actuators are not operative, so, in the air, aerodynamic forces may move the bogies into an untilted position (ground mode)

[John H Watson]

Here is some of the air-ground logic.

PSEU Air-Ground Logic

Some systems use the air-mode output of the PSEU. Some systems use the ground-mode output of the PSEU. Some systems use both outputs of the PSEU. Some systems use an output of the PSEU which doesn't use this logic, so, as Avi says, each aircraft system has to be looked at separately.

[Hardy Heinlin]

Sylvain, now I understand your configuration. As I described above: The PSEU will not set the on-ground mode because it receives a "low pressure" signal from HYDIM 1; this invalidates the "tilted" status. In this situation it will validate the "tilted" status only when HYDIM 1 is unpowered. So if you pull the HYDIM 1 CB at F3 on P7 while the gear is tilted, the PSEU will promptly set the on-ground mode. Well, that's how PSX models the PSEU system and its dependencies. We checked every bit during PSX development; did we miss any parameter that bypasses the HYDIM signal?

Reverser operation requires an engaged on-ground relay to determine "not in flight"; it will not tolerate a relaxed in-flight relay due to an unpowered PSEU to determine "not in flight"; that would be too risky. Disabled reversers on the ground are less dangerous than enabled reversers in flight. Each system uses that relay which provides the smallest risk in case of a PSEU failure. Some systems must remain operative in flight, some others on the ground. And: Some inhibits must remain intact in flight, some others on the ground.

John, in my code the nose gear can set the on-ground mode only if HYDIM 1 or 4 sends a "low pressure" signal. I probably shouldn't have written "nose gear related". It's main gear related. I consider the nose gear check a last resort in case the main gear sensor system failed, i.e. the ground mode will be set upon nose gear touchdown then.


Cheers,

|-|ardy

[Hardy Heinlin]

Ha!

Got it.

It worked in PSX versions prior to 10.1.0-beta5, right?

In 10.1.0-beta5 I fine-tuned the pitch attitude on the ground for certain abnormal gear configurations. In the scenario above the pitch is now 1.2° instead of 0.0° due to certain gear strut compression effects. This affects the nose gear on-ground detection. Beta 5 thinks the nose gear is not on the ground.

Normally, in this case, you should get the on-ground mode when the nose gear has landed. No need to pull HYDIM CBs.

I'll fix this in beta 6 ...


|-|

[Hardy Heinlin]

Fixed in PSX 10.1.0-beta6:

http://aerowinx.com/board/index.php?topic=3858.0


Cheers,

|-|ardy

[Jeroen Hoppenbrouwers]

Talk about unexpected side effects...