If my memory serves me right, we had a 763 that was losing centre fluid on every sector. When it was topped up on the ground and tested, no leak could be found. There was no evidence on the airframe of where the fluid was leaking from and eventually it went into the hangar for further investigation.

It was jacked up and gear swings carried out. In the end, it was found that the tailskid was leaking, but only when "in the air". Apparently, hydraulic pressure is applied in flight to keep it retracted.

This was a few years ago, but I think the details are correct. Certainly foxed everyone for a while at the time.

Well, actually, with alternate gear extension and no center hydraulics, you normally end up with the gear and gear handle down and the tailskid up at the completion of the checklist.

The handle does nothing for the alternate extension itself without hydraulics but you lower the handle after the gear is down to avoid the Gear Disagree (and GPWS/Config) warning. It is normal to have a Tailskid light and EICAS message with alternate gear extension since the tailskid remains up but the gear handle is down.

That's how it 'should' work. I agree with everyone who is puzzed about the gear up and the tailskid down. Did the tailskid never retract on takeoff? And was the hydraulic leak at the tailskid itself as in the incident that 763 Jock remembers? Maybe the fluid was pumped out after the gear came up through the tailskid retract mechanism, perhaps?

Not just could, but did - resulting in the loss of a Vickers Valiant on 11 May 1956.

Read about it here: ASN Aircraft accident 11-MAY-1956 Vickers Valiant WP202

763 jock

Thanks, classic leak in the up lines scenario.....foxed many people on manys an occasion.

By implication therefore, it would appear if there is no pressure the tail skid may droop. (I assume it's a fairly heafty bit of kit)

Airbubba

surely that depends if there's pressure available or not? :confused: (even with pumps off, the pressure may still take some time to dissipate)

If you don't open the return lines won't you create the potential for a hydraulic lock? :confused:

So back to my original question; What holds the tail skid up???

sounds like an oversight in your sim scenario to me.

The MLG does not have its own uplock viz 744 but the MLG doors do.However,the gear does not rest on the doors in flight.Now as it looks like the doors were released via the ground ALT release,we may have an issue with the electrical ALT actuator.Perhaps the best option is to wait and see the preliminary report,after all,we can only make educated guess's.I am very sure that this one will not take very long to find the cause.

Well, them's the procedures from Mr. Boeing. With loss of center hydraulics you do alternate gear extension with the gear handle off, then put the handle down when you get (hopefully) three green. And the tailskid does not extend, it is held up by PFM. You also hit the Reserve Brakes and Steering switch in my purely fictional Microsoft flight simulator version of the procedure which I downloaded from BitTorrent.

That's my story and I'm stickin' to it.:E

It may work different on the -400 (or the -200;)).

How could you tell I wasn't really a pilot?:{

It has no uplocks, it is not pressurised in flight, what holds it up?

I couldn't...thats what worries me. :\

I could tell you weren't an Engineer though.

Rem; Simulators and Lawyers understand checklists.....aeroplanes don't. :cool:

The B767 main gear do rest on the gear doors when gear handle in off and no pressure in the system.

The main landing gear door supports the weight of the retracted main gear when the landing gear system is depressurised.
More precisely, there is a gear support attached to each main gear door. When the door closes the gear support folds under and locks the main gear. The door is held closed by the door uplock hook. When the landing gear system is depressurised the weight of the gear rests upon the gear support.
During alternate extension, the uplock hook is released mechanically and the weight of the gear pushes the door open.
It is a little more sophisticated than the tyres resting on doors :)

WRT to the tailskid and what holds it up. C HYD pressure does.
Incorrect, Unlike the gear, it IS pressurised in flight. The retract pressure is maintained by the centre system with the tail skid retracted as the control valve is in the same position with the landing gear lever UP or OFF.

In Polish: Awaria Boeinga mog

Data in this article seem to be quite precise, so I suppose it's from reliable source (reference to unofficial information from State Commission on Aircraft Accident Investigation) and worth quoting.

According to the DFDR the failure of C HYD occured just after the retraction of landing gear during departure from Newark, or even at the same time. It's impossible to point the exact moment of failure as this data is recorded only once every 64 seconds. The same minute the gear was retracted there was a sudden drop in hydraulic pressure from 3000 to 400 PSI and 90% of the fluid (3 out of 40 gallons remaining) apparently leaked out to some "hatch" in the aircraft.

Indeed, the circuit breaker had been found tripped by LOT engineers, but it was possible to press it into proper position during the first attempt and that is how the landing gear was extended on ground. Finding out the exact moment when the CB tripped might be tricky (no such data on DFDR), but hopefully entirely possible using the memory of certain individual units (any ideas on this one?).

Before jumping to conclusions about the circuit breaker not being reset in flight rememeber that -something- caused it to trip in the first place and
whatever that something was may well have been jarred loose during the landing.

JB6: BTW, this is meant for general consumption, not at all saying you implied anything - or that the landing was not amazingly smooth given the conditions.

The CB that controls the ALTN landing gear is located on the sidewall behind the F/O's seat. As such, it is in a vulnerable area with regards to things like passing nav bags, overnight bags and the usual stuff that lives on the floor. I have come across "tripped" breakers in this area during pre-flight on more than one occasion. It is also close to a 115V AC outlet.

It would be very bad luck if it was out due to a bag moving in flight or being snagged by a cable. Unlikely, but not impossible.

Fair enough!;) Anyway, it may defy logic but I think I have correctly described the gear handle operation in the alternate gear extension checklist. Do you have another version of this checklist, perhaps?

Well, we pretty much have to go by the book and the checklist from the operator side of things. And I agree with you that some of this stuff is fit for an attorney. The FAA and Boeing have been trying to go to 'integrated checklists' for systems malfunctions, e.g. if you lose C HYD, you will have to do alternate gear and flap extension so the procedures are rolled into one big checklist rather than having you hop around the QRH.

Earlier, even though you knew normal operation would not work, you would position the gear or flap controls, get a 'Disagree' light and it would lead you to a separate checklist with double negatives and other challenges for each inoperative item on the lost hydraulic system.

The new integrated checklists have a laundry list of other checklists that you don't do, e.g. the 'Tailskid' checklist with loss of C HYD since it is an expected indication with the gear handle down and no center pressure.

Thanks for this explaination, I appreciate it.:ok:

I was actually referring to the gear being unpressurized during flight but I didn't realize that the tailskid stayed pressurized, it sure increases the possibility that the leak was there it seems.

Years ago I did a crossing with the tailskid MEL'd. Seems like we took a couple of thousand pound performance hit on the runway limit weight and maybe a couple of percent on the fuel burn. The tailskid was locked down, maybe it was pinned somehow, it's been quite a while since we did this.

flaps 20 or more
 

Thanks for polish press link above given by jamesbond6


I have a question :


Flaps geometry as shown on the link shows aft flap gapped with the main flap .


As this aft flap is moving as soon main flap moves beyond 20 ° , and as alternate flaq sequence is limited to 20 ° , I see quite an technical contradiction ..

Any comments ?

The centre hyd system failure QRH calls for ALTN flap and gear. If this runs to plan, flap 20 is used for landing. However, if you look at the "Gear Disagree" QRH, you are advised to land on available gear. In that case, flap 30 is required to reduce the Vref.

Perhaps I was a little simplistic in my explanation, Roo is quite correct.In fact there have been cases of the MLG hanging up on the doors during swings testing the Alternate Extension.Fortunately we have never encountered this doing swings.As they say, a picture is worth a thousand words,I wish our MLG bays were that clean too!

006,

I'm just SLF, but retracting the gear on the ground doesn't seem like a very good idea, somehow. :hmm:

New Developments
 

The hydraulics failure was detected 30 minutes after takeoff and the pilot considered returning to Newark. But, after contacting the HQ the "technical engineers" on the ground made the decision that they cross the ocean.

The decision was probably correct but it brings us back to the usual post-commie block topic of who makes the final decision on issues like this one - the pilot or some ground "specialists" or politicians breathing down your neck.

Its standard practice to function check gear operation on the ground after a gear main component change,the aircraft is either on jacks or jacks and wheel pit.As for worrying about retraction whilst standing on main gear,that is why you install "the pins" in the downlock links and sign for it in the Tech Log.Should someone select "gear up",the pins will not allow the downlock links to fold.

@ Sadpole

Consulting one's own airline in such cases has nothing to do with communist mentality, as you suggest. In fact it is standard practice in all airlines I know - the tech staff on the ground have better knowledge of the aircraft's systems than the flight crew, access to documentation etc. and usually are able to provide invaluable advice to the crew. However, the final decision lies with the commander and he may continue, or divert wherever he sees fit. Period.

Besides, all the data presented so far suggests that the decision to continue to the destination had no impact on the (in)ability to extend the gear and in fact, had been the most sensible in the circumstances and in line with the basic concept of triple-redundant systems (such as 767 hydraulics), which allows to continue safe flight in case of single failure...