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GearDown&Locked
16th May 2006, 10:37
I was reading the 85 747 JAL crash report and noticed the fact that they entered in a state of vertical oscillation and from the timeline of the cvr transcript they were in this state for about 30 mins or so.
Would it be possible to stablise an A/C in this condition? I know we're talking about a big and heavy 747, but would it be recoverable on, say, a 737? Thanks.
GD&L
(post title edited - incorrect at source :hmm: aviation-safety.net (http://aviation-safety.net/database/record.php?id=19850812-1&lang=en), although I can't change the thread title, my apologies)

chornedsnorkack
16th May 2006, 10:57
I was reading the 85 747 JAL crash report and noticed the fact that they entered in a state of vertical oscillation and from the timeline of the cvr transcript they were in this state for about 30 mins or so.
Would it be possible to stablise an A/C in this condition? I know we're talking about a big and heavy 747, but would it be recoverable on, say, a 737? Thanks.
GD&L
I do not think big and heavy matters much. Actually, it might well be harder on a small plane... The Sioux City DC10 also had phugoids and crashed exactly because of phugoid on landing. Not sure about how the shot-down DHL A300 managed.

What happened was that all three planes suffered loss of all hydraulics and therefore all hydraulic control surfaces froze. The remaining control was throttles.

Sioux City DC10 had 2 throttles for control because the middle engine had exploded, while the DHL A300 had 2 throttles because there only were 2 engines to begin with. The JAL 747 would have had 4 throttles, so in principle slightly more degrees of control... but exactly how does inboard/outboard thrust affect a 747?

GearDown&Locked
16th May 2006, 11:45
So my guess is with reduced elevator capacity it is almost impossible to control such movements, and the end is pretty predictable :uhoh:
Thanks for the reply chornedsnorkack.

GD&L

Milt
16th May 2006, 12:57
Phugoid expectations.

A phugoid in pitch with stick fixed or controls locked will usually have a lengthy period exceeding a minute. If there remains something that can be imposed on the phugoid such as changing thrust then the phugoid may be altered. But the problem is at what stage does one insert the change. It is unlikely to be instinctive and the odds are that the change will be initially inserted at the wrong time thus magnifying the amplitude. Given enough time one may get close to determining the optimum time to insert the change to dampen out the phugoid.

If one is able to increase the pitching stability by moving the cg forward with fuel transfer or pax movement one could expect some degree of reduced amplitude.

There will likely be a particular average IAS where the phugoid will be around a level altitude.

Comments?

Old Smokey
17th May 2006, 02:36
it is almost impossible to control such movements, and the end is pretty predictable

Not so, as long as the engines have a thrust line somewhat above or below the C of G (e.g. B747, DC-10, A300), then thrust variation will provide for a reasonable degree of pitch control. This was pretty much proven in the Sioux City DC-10 accident.

If the thrust line is at or close to the C of G (e.g. B727, MD-90, F28, many business jets), then little or no useful pitch variation occurs with thrust change. Thankfully, for the examples that I've given, Manual Reversion, or Primary Manual control is available in all cases.

Regards,

Old Smokey

Rainboe
17th May 2006, 10:13
A 737 always has manual reversion in the event of total hydraulics failure, though this brings a whole new set of problems, but at least you have an element of control. My own opinion of manual reversion from my experience in the simulator is that it is obviously far better than nothing, but your survival is not assured. It's a terrible way to fly a plane, aiming to contact terra firma at a survivable rate!

After the JAL incident, there was some discussion on the 747-200 about being able to at least get some pitch authority back in a total hydraulic failure scenario by selective movement of the trailing edge flaps using selective selection of inboard/outboard TE flaps and alternate flap extension- inboard flap extension giving a small pitch up, and outboard a pitch down. Doesn't sound practical, but it works- in a very small fashion, we tried it in the simulator one night. Whether you could encounter phugoids with this is questionable- it is very, extremely, slow. But remember, you have nothing else but power and CofG movement- beggars can't be choosers, so you have to use what you can. I think even this ability was lost on the 747-400 and it's not possible. I think it is accepted a total hydraulics failure is virtually non-survivable.

ORAC
17th May 2006, 10:49
Do a search and read up on Propulsion Control (http://www.nasa.gov/lb/centers/dryden/history/pastprojects/PCA/index.html). Humans might not be able to do it, but the computers can. After the trials some airlines showed some interest in having as an option in their aircraft, but I do not know if it ever entered service with anyone.

chornedsnorkack
17th May 2006, 11:54
I think it is accepted a total hydraulics failure is virtually non-survivable.
All three total hydraulic failures described had survivors...
Is the 727 centre engine thrustline at the same height as those of side engines?
And how far are the thrustlines of the inboard engines from the thrustlines of outboard engines on 747?
Looking at roll and yaw, an airplane has 2 separate systems of aerodynamic controls for those movements. Ailerons and rudder.
And then, for multiengine planes, there is asymmetric thrust. So 3 different systems to create lateral forces... Unwanted asymmetric thrust from asymmetric engine failures can be countered by aerodynamic controls. And then again, an airplane can be steered by two throttles giving controlled asymmetric thrust, with both rudders and aileron frozen, as done on DC-10 and A300.
But on a 747, there are more than two throttles. Are there any pitch effects of changing the thrust distribution between inboard and outboard engines (while keeping total thrust constant and symmetrical)?

Rainboe
17th May 2006, 12:12
The outboards, although higher than the inboards, are still below the CofG, so I think their effect is still lower than the inboards, but in the same direction. I am sure the JAL pilots used thrust to try and counter the phugoids without success.

Old Smokey
17th May 2006, 13:15
chornedsnorkack,

Is the 727 centre engine thrustline at the same height as those of side engines?

YES

Regards,

Old Smokey

GearDown&Locked
17th May 2006, 15:40
ORAC, thanks for the link to a very interesting system, the PCA. It would be very usefull as a backup and not too difficult to implement. But without a direct recommendation from the Aviation Authorities, I'm afraid it will remain at being just a very good idea. :suspect:

It is unlikely to be instinctive and the odds are that the change will be initially inserted at the wrong time thus magnifying the amplitude. Given enough time one may get close to determining the optimum time to insert the change to dampen out the phugoid.

Milt, that was also my understanding about compensating / opposing the a/c movement with inverse action. Ones input could jeopardise the situation if acting at the wrong wave sequence. Are this type of actions trained very often?

I think even this ability was lost on the 747-400 and it's not possible. I think it is accepted a total hydraulics failure is virtually non-survivable.
rainboe, why should it? Why not use electric or pneumatic backup sources if the engines are still running? odd it seems, but who am I to disagree with that.

Old Smokey, having read ORAC's link, we will find a very good example of an MD-11 controled by thrust power only (although trough that computerised engine control system named PCA). Given its peculiar CofG, it is very interesting how it worked so well.

GD&L:ok:

Rainboe
17th May 2006, 17:00
At some stage, you have to accept you can't back up all systems indefinitely, and still be able to carry a payload on an aeroplane. The design of the 747 has been refined over the years to make redundancy superb. Four hydraulic systems, all feeding various equipment, never altogether, pipework kept apart, all four systems able to operate even if down to one engine, and indeed, able to supply pressure from a windmilling engine, high pressure air and engine driven pumps in each system with backup sources of operation for vital equipment- it's got to be as near foolproof as you can make. to add the complexity of further alternate systems is not justifiable. The SFO Pan Am take-off incident and multiple hydraulic failure was exceptional, and didn't lead to total failure. The system is good, you have to look elsewhere for likely fatal failures- i.e. floor collapse in pressurisation failure breaking control runs.

glhcarl
18th May 2006, 03:31
.....
What happened was that all three planes suffered loss of all hydraulics and therefore all hydraulic control surfaces froze. The remaining control was throttles.....

When hydraulic pressure is removed from a powered control surface, it does not freeze, it goes to an aerodynamically faired position.

galaxy flyer
18th May 2006, 04:12
On the C-5, with all hydraulics unpowered, it had a phugoid oscillation. It could be suppressed by applying and reducing power at the right times; my memory is power off at the top and power up at the bottom. In the sim, most guys could handle the phugoid, once they a handle on the period which was quite long and induced overcontrol.

Pitch could be controlled by adjusting CoG (dump fuel, move cargo/pax) and using the ground spoilers (override the ground-only restriction). Again, in the sim, one could usually have a controlled crash on the airfield. They did a very loosely controlled crash in Saigon and, like Souix City, many survived.

GF

glhcarl
18th May 2006, 04:35
Pitch could be controlled by adjusting CoG (dump fuel, move cargo/pax) and using the ground spoilers (override the ground-only restriction). Again, in the sim, one could usually have a controlled crash on the airfield.
GF

How do you get the ground spoilers to deploy when you have no hydraulic power?

Feather #3
18th May 2006, 05:57
Rainboe,

Did a similar trial in the B742 ourselves. Took a bit to sort out at 10,000ft with excursions down to 5,000ft, but landed [we chose ditching in fact] sortof OK.

The B744 is a worry, but if you have it sorted in the brain, you can use the alternate flaps but trip the correct CB's as needed to control inboard/outboard movement. However, not something I'd like to try in a hurry!!:uhoh:

G'day

Rainboe
18th May 2006, 09:04
With all that to handle in a 744, and 2 pilots only, I think it would be one problem too many! It wasn't something I ever researched in the 744- I think that planes hydraulics are perfect, it's reached the degree of refinement where other systems maybe examined instead.

DozyWannabe
18th May 2006, 11:38
I am sure the JAL pilots used thrust to try and counter the phugoids without success.
As I recall, they got pretty close to mastering it, but unfortunately their location meant that they ran out of time and altitude before they had figured it out completely.

The fact that they kept it aloft for around 30 minutes, despite the fact that this was pre-UAL232 (the incident that proved that thrust control was possible), that for the first 15 minutes or so they were high enough to suffer hypoxia, plus they never fully understood just how badly damaged their aircraft was, suggests to me that they could have done it in more fortuitous circumstances.

chornedsnorkack
18th May 2006, 12:14
At some stage, you have to accept you can't back up all systems indefinitely, and still be able to carry a payload on an aeroplane. The design of the 747 has been refined over the years to make redundancy superb. Four hydraulic systems


The problem with hydraulics seems demonstrated by 3 cases of airliners suffering total hydraulic failure despite redundancies.

A300 had a rocket in one wingtip. The aileron was served by three redundant hydraulic systems... so they all failed, and the aileron at the other wingtip as well as the aerodynamic controls of tail, though mechanically intact, became useless. DC-10 had engine shrapnel in tailfin... again, it was served by all three redundant hydraulic systems, so they all duly failed and ailerons, flaps et cetera became useless. B747 has rear bulkhead failure... again, it broke all 4 hydraulic systems, so that e. g. ailerons at the end of, mechanically intact, wings became useless.

How do airplanes deal with this kind of multiple common-cause failures?

Rainboe
18th May 2006, 16:51
What you describe does not sound like hydraulic redundancy. Only fatal wing or tailplane damage could take out all 4 hydraulic systems- wingtip damage could only take out 2 systems. Each side of the tailplane is only served by 3. Only 3 hydraulic systems go to the fin. It should not happen to the 400 series. Engine damage, even traumatic, should only have very limited damage potential to the total hydraulic systems. That is why I said rather than try and improve on that system, other areas of risk need to be examined- floor collapse, rear bulkhead etc.
It is the designer who is the chief weapon against such common cause failures. Later developments incorporate a lot of improvements. The 737 hydraulic system and the way it was modified from 100/200 series to all subsequent models shows this.

DozyWannabe
18th May 2006, 17:16
IIRC do the hydraulic pipes not go through the ceiling rather than the floor in the 747? I thought that was why the crew were so concerned about hydraulic loss during UAL811...

Also, I think that JAL123 and UAL232 caused the development and retrofitting of fuses to the hydraulic systems in passenger transport. The DHL A300 was built in '79 so may not have been modified in this manner. Having said that it lost a lot more than a wingtip.

AirRabbit
19th May 2006, 00:26
The problem with hydraulics seems demonstrated by 3 cases of airliners suffering total hydraulic failure despite redundancies.
A300 had a rocket in one wingtip. The aileron was served by three redundant hydraulic systems... so they all failed, and the aileron at the other wingtip as well as the aerodynamic controls of tail, though mechanically intact, became useless. DC-10 had engine shrapnel in tailfin... again, it was served by all three redundant hydraulic systems, so they all duly failed and ailerons, flaps et cetera became useless. B747 has rear bulkhead failure... again, it broke all 4 hydraulic systems, so that e. g. ailerons at the end of, mechanically intact, wings became useless.
How do airplanes deal with this kind of multiple common-cause failures?
The most common outcome is what happened to the JAL B747.

galaxy flyer
20th May 2006, 02:02
How do you get the ground spoilers to deploy when you have no hydraulic power?

You made me think, glhcarl! The answer is, the flight manual procedure using the spoilers was for loss of systems 1,2,3, in which case all pitch control was lost. You could use the ground spoilers, powered by remaining system 4 to make rough pitch corrections. Systems 1,2,4 caused loss of roll control.

Cannot understand why Lockheed did not make the flight controls symmetrical down to one system--easily doable, the L-1011 was.

GF

glhcarl
20th May 2006, 04:32
You made me think, glhcarl! The answer is, the flight manual procedure using the spoilers was for loss of systems 1,2,3, in which case all pitch control was lost. You could use the ground spoilers, powered by remaining system 4 to make rough pitch corrections. Systems 1,2,4 caused loss of roll control.

Cannot understand why Lockheed did not make the flight controls symmetrical down to one system--easily doable, the L-1011 was.

GF
I have no experance with the C-5 but I assumed it had the same basic hydraulic system design the L-1011 did. Losing three of the four L-1011 systems, the most you could lose is yaw control.

Ignition Override
20th May 2006, 06:46
Gear Down and Locked:

Aside from the phugoid cycles, why did Boeing design the 737-200 to use hydraulic pressure on all primary controls, regardless of whether manual reversion is safe? The Douglas DC-9 has none, except for a little rudder boost from the right h. system, and accum. pressure available for the elevator control tabs to help recover from a (t-tail) deep stall. Without the right h. system (slats and flaps should extend at a slower rate etc), 135 knots or Vref + 5, until landing assured. No big deal.

Maybe Boeing planned to stretch the fuselage?
MD never had it on the MD-83 primary controls, any more than on the DC-9, from what friends tell me.

Milt
20th May 2006, 08:02
Trim only approaches and landing.

Trying to land using trim only on some types can develop some good phugoids mainly because one tends to overcontrol.

Best if you try it on a simulator first!

galaxy flyer
20th May 2006, 23:24
glhcarl:

From the guys that flew the L-1011, a lot was similar and recognizable, but the C-5 was definitely the Mark I version. OTOH, I lost a number of systems, one at a time. The book answer for loss of 2 was slow to 200 KIAS, land as soon as practical. 200 Kts approximated the correct stab time for landing. Friends who have landed with 2 out, said all was normal, until close to the ground when you suddenly found out how little authority you have, depending on the systems out.

GF