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Plughoid oscillations

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Plughoid oscillations

Old 16th May 2006, 09:37
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Phugoid oscillations

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 aviation-safety.net , although I can't change the thread title, my apologies)

Last edited by GearDown&Locked; 16th May 2006 at 10:55.
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Old 16th May 2006, 09:57
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Originally Posted by GearDown&Locked
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?
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Old 16th May 2006, 10:45
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So my guess is with reduced elevator capacity it is almost impossible to control such movements, and the end is pretty predictable
Thanks for the reply chornedsnorkack.

GD&L
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Old 16th May 2006, 11:57
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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?
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Old 17th May 2006, 01:36
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Originally Posted by GearDown&Locked
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.

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Old 17th May 2006, 09:13
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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.

Last edited by Rainboe; 17th May 2006 at 09:50.
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Old 17th May 2006, 09:49
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Do a search and read up on Propulsion Control. 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.
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Old 17th May 2006, 10:54
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Originally Posted by Rainboe
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)?
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Old 17th May 2006, 11:12
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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.
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Old 17th May 2006, 12:15
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chornedsnorkack,

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

YES

Regards,

Old Smokey
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Old 17th May 2006, 14:40
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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.

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
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Old 17th May 2006, 16:00
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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.
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Old 18th May 2006, 02:31
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Originally Posted by chornedsnorkack
.....
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.
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Old 18th May 2006, 03:12
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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
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Old 18th May 2006, 03:35
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Originally Posted by galaxy flyer
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?
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Old 18th May 2006, 04:57
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Question

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!!

G'day
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Old 18th May 2006, 08:04
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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.
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Old 18th May 2006, 10:38
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Originally Posted by Rainboe
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.
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Old 18th May 2006, 11:14
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Originally Posted by Rainboe
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?
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Old 18th May 2006, 15:51
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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.
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