UNREALISTIC HYDRAULIC SYSTEM FAILURE!

[Guest]

Anyone ever wondered if they could do as good a job as the UAL crew did in bringing their crippled DC-10 in at Sioux Gateway a few years ago? Well, so did I. I set up a .83 cruise at FL370 with the PIC 767 and shut down all of the hyd. systems to simulate the kind of complete failure experienced by the UAL crew that day. I had ABSOLUTELY no control of the airplane! Worse than that though, the nose started rising abruptly and and the a/c soon stalled and started falling like a leaf toward the ground. I'm wondering if anyone has successfully simulated the effect that I was going for here. Maybe there's an adjustment that can be made somewhere to keep the airplane from going into an immediate climb? I don't make it a habit of putting this great a/c through such abuse but it's claim to fame is it's accuracy in normal procedures as well as failures so I thought I'd bring it up. Thanks.

[Guest]

Hi, Jeff!Check RAT is unlocked. I believe it only deploys automatically in a dual engine failure. I think I go and try that myself...Groetjes, http://www.grillsportverein.de/smilies/con...doom/bounce.gifDiederik Krauwer - EHAM

[Guest HPSOV]

Took off out of Sydney, climbed straight ahead to 31000ft then failed all 3 hydraulic systems.The immediate effect of this was a huge nose pitch up resulting in a stall, followed by a nose pitch down. This oscillation repeated with the aircrafts altitude going up and down by about 8000ft each time. This phenomenon is known as a long period oscillation or a "phugoid" (for all you aerodynamacists out there), it is a result of an aircraft having positive longitudinal dynamic stability (which is a good thing). As a result of that when the nose pitches down in the stall, the tendancy as the speed builds is for the nose to pitch back up, and when the nose pitches up, the tendancy as speed drops is for the nose to drop again.I then turned around 180 degrees using assymetric thrust, I found it was much more effctive when used at the top of an oscillation, as it tended to simply yaw the aircrafts heading round, instead of putting on a large bank angle.Now I was just making small heading changes to line up with the runway (still some 30nm from touchdown), heading control was a lot easier than I thought it would be.When I got to about 15000ft I started running flap and dropped the gear, to get into landing config as early as I could (remember alternate gear/flap will have to be used).When I got to about 5000ft I started working on damping out the oscillations by applying full thrust at the top of the "wave" to provide a pitch up while the aircraft was pitching down, and idle thrust when it started to pitch back up. Now its just a matter of judging your height/distance from the runway, and trying to make sure you hit the ground at the bottom of one of the oscillations when your v/s is nearly level. Using thrust changes all the time to influence the aircrafts attitude.I eneded up hitting the runway (a little right of centreline hehe) about halfway down with a v/s of about -800fpm.In all I was surprised by how easy it was, and thanks Jeff for giving us all a bit of a challenge!!

[Guest]

Wow! I'm impressed that you were actually able to get the airplane on the runway like that. Fine aviating indeed! My point though, is that the behavior of the a/c just after the failure of the the 3 systems doesn't seem accurate. I consulted with several pilots yesterday about the issue and their reactions were unanimously the same. If the airplane is trimmed for straight and level flight and the hydraulic systems are turned off, there shouldn't be an abrupt reaction like we experience in PIC. The reason being that nothing has happened to affect the aerodynamics of the a/c at that point. I'm no expert on hydraulics but it seems to me if anything, the nose would initially pitch down in response to the control surfaces aligning themselves with the airflow around them. It could be that we're all wrong and the response of the PIC 767 is 100% accurate but I'd have to have someone convince me. Anyone with knowledge in such matters care to join in on this topic?

[Guest HPSOV]

We need Ian Ridell to delve into the maintenance manuals for us on this one :-)But I think at the end of the day there is no way of predicting what will happen when ALL hydraulic pressure is lost to all control surfaces.Will the elevators float up or down? And more importantly will the stabiliser remain in its last position, or will it too be affected by the relative airflow.I know that when hydraulic pressure is removed from an aileron in-flight it will float up, with the trailing edge sitting around 4 inches above the neutral position.

[Guest Ian_Riddell]

"We need Ian Riddell to delve into the maintenance manuals for us on this one"Hmmm... Not sure about this, HP.... I think my Airframe/Engine colleagues would have a fit if I started looking into this sort of stuff ( demarkation issues ) :)Notwithstanding, I doubt that the Boeing MM would tell us anything about this stuff.Without hydraulics, the control surfaces would be pretty much at the mercy of gravity and airflow (At cruise speeds, however, gravity is not going to have much effect on the final positions of the elevators and ailerons... as they are relatively light for their size and some (if not all) of them are, to some extent, counterbalanced).Can we always presume that the aircraft is autotrimmed so that elevator is faired with the stabilizer? (I know that during Land 2 on a 747-400, for example, the elevator is not faired. The stabilizer is positioned several extra units nose up and the elevator actually has to do some (fly down) work to maintain the glidepath. This way, if the autopilot trips off in the final moments of landing, the elevators will not provide the fly down effect... and the nose will pitch up (as it would do normally during flare). Presumably, the 767 would have a system similar to this (?). Are there any other phases of flight where something akin to this happens (e.g during high speed flight)?It's a while since I studied the basics of trimming, but I recall hearing the expression "mach trim"... If autotrimming is simply to keep the aircraft flying along the intended path with the elevators faired, why would there be a need for something like "mach trim"? Does mach trimming do something different to the horizontal stabilizer (as during Land 2? We might have to take this question to the PPRuNe site ;-)).Sorry, not much help at the moment (more questions than answers).Cheers.Ian.

[Guest]

HPSOV,>When I got to about 15000ft I started running flap and Now try it without the flaps ;)http://www.panix.com/~jac/aviation/haynes.html"Well, on July 19th, Murphy's Law caught up with us, and we did lose all three systems. And as a result, we had no ailerons to bank theairplane, we had no rudder to turn it it, no elevators to control thepitch, we had no leading-edge flaps or slats to slow the airplane down, no trailing-edge flaps for landing, we had no spoilers on the wing, to help us get down, or help us slow down, once we were on the ground.">I ended up hitting the runway (a little right of centreline hehe) >about halfway down with a v/s of about -800fpm"That concrete is 12" thick, and the hole is 18" deep. You normallyland the DC-10 at approximately 140 knots. We were doing 215 kts andaccelerating. You normally touch down at about 200-300 feet per minute at the most, as a rate of descent. We were doing 1850 feet perminute. And increasing."

[Guest]

Thanks Ian. You've given us MORE to think about. I haven't tried shutting the systems down at a slower speed. Didn't realize all of that was going on back there depending on the phase of flight. Looks like some more flight testing is in order.

[Guest Iz]

Ian, the Mach trim is to counter "mach tuck", which is the tendency of the aircraft to nose down when reaching high mach numbers. I know that in the 737-200 there's a seperate switch and test for the Mach trim on the overhead panel, meaning that since it's Mmo is something like or .74, the mach tuck phenomena will occur somewhere in that region.On the wing, a non-powered aileron would indeed move up during flight since the wing produces lift (hopefully) and there's a low pressure area above the wing.On a trimmed stabilizer, I believe it depends on the aircraft, center of gravity, position of the main wings and stabilizer to see if it either requires downforce, neutral or upforce to keep the aircraft steady.I believe on a non-deflected, trimmed stabilizer on the 767 it produces downforce. If the stabilizer just fell off, it would result in a nose down situation. Therefore, there should be an area of low pressure underneath the stabilizer, causing a non-powered elevator to move down, causing a nose down moment.Just speculating though.As for the accuracy of 767PIC, the aerodynamics depend on the aircraft file (not the panel) of course and since Eric is not a Boeing test pilot, there's no way to have "THE CORRECT FORMULA" for the 767 or any other plane. I don't even think a Boeing test pilot would know it.Anyway, the 767 would in principle not get into such a situation as the United DC-10, since the hydraulic lines don't all run through a single area like that (let alone close to an exploding engine!). The only situation where this could happen is a leak in every hydraulic reservoir (all 3) resulting in the loss of all hydraulic fluid.And in an emergency, the flaps/slats can be extended electrically.

[Guest]

And of course, don't forget the RAT that can be deployed in case of complete hydraulic pumps failure. The RAT provides sufficient hydraulic pressure to move the flight controls. The RAT is also deployed automatically if both engines fail in flight.Regards,Laurent Crenier_____________________________PIC Panel co-designer

[Guest Ian_Riddell]

>Ian, the Mach trim is to counter "mach tuck", which is the >tendency of the aircraft to nose down when reaching high >mach numbers. I know that in the 737-200 there's a seperate >switch and test for the Mach trim on the overhead panel, >meaning that since it's Mmo is something like or .74, the >mach tuck phenomena will occur somewhere in that region. Assuming the 737-200 has an autotrim system, Iz.... how would a Mach trim system operate at the same time as an autotrim system? (or doesn't it?). If a mach trim system added/subtracted trim from autotrim trim, wouldn't the aircraft leave the intended flight path? (The 747-400 has an active trim system even when the A/P is disengaged. It provides better speed control during manual flight at lower speeds... but I can't quite see the 737 having a system like this for high mach speeds). My training notes don't go into the purpose/function of autotrim other than to say that if the A/P disengaged and it is trimming the aircraft, the aircraft would not go out of trim if the A/P disengaged (except in the case of LAND2). This would suggest that with normal hydraulics available and the controls spring loaded to the neutral position, the aircraft would remain in straight and level flight (if it was in straight and level flight beforehand). No hydraulics may be a different matter however.>Therefore, there >should be an area of low pressure underneath the stabilizer, >causing a non-powered elevator to move down, causing a nose >down moment. >Just speculating though.I came to a similar conclusion when you reminded me of the upside down airfoil... The 737 and 747 also have these :-)Interesting stuff.Cheers.Ian.

[Guest Ian_Riddell]

>And of course, don't forget the RAT that can be deployed in >case of complete hydraulic pumps failure. The RAT provides >sufficient hydraulic pressure to move the flight controls. >The RAT is also deployed automatically if both engines fail >in flight.I see that if a PIC simmer were (masochistic enough) to also simulate a Centre Hydraulic System leak, you would lose the RAT, too.... (The RAT is provided with hydraulic fluid from the C system reservoir). :(BTW, according to the Maintenance Manual..."The (RAT) hydraulic pump supplies 11.3gpm (gallons per minute) at 2140psi. The pump flow is controlled by the output pressure. At 3025psi, the pump delivery flow is zero".I notice that the RAT produces 3000psi in PIC over a wide range of airspeeds.... Is this a typical value? (Experts! :-))Thanks.Cheers.Ian.

[Guest Iz]

Well to be honest, I only flew a 737-200 simulator a few years ago, had some brief instruction but that was it. From my 757 AOM: Automatic Trim.The stabilizer is controlled automatically by the autopilot or by a Mach speed trim system when the autopilots are not engaged. The Mach speed trim system improves speed stability by trimming the stabilizer as airspeed changes. Electric, backup or autopilot trimming inhibits the Mach speed trim system.Automatic stabilizer trim uses only one trim control module and trims at one-half the electric or alternate trim rate.So I conclude that the Mach trim only operates when flying manually, the autopilot trimming incorporates the same airspeed logic when an autopilot is engaged. The 757 flies extremely stable even when accelerating or decelerating (I like to hand fly it as much as I can), I probably should ascribe this to the Mach trim. I heard from captains who also flew 737-200/300/800 that that plane requires much more active trimming.Anyway, would this be the same as the 744 autotrim during manual flight? I would guess so. I don't see how the 744 would have an effect directly on speed control there, since autotrimming will only be active during acceleration or deceleration (it's not an Airbus).It does not compensate trim for flap changes.Iz

[Guest]

Well, anyone who can land any big airplane under these conditions, real or simulated, is doing a great job indeed! And he is a test pilot at that moment, since even the manufacturers don't test this in the real airplane, for perhaps obvious reasons.Even in the big airline sims I'm not certain that the effects are all that accurately modeled, since there is little real data to use. As some point out, controls will have varying and perhaps unpredictable behaviors absent their normal hydraulic pressure. Then, probably the biggest challenge would be the effects of the atmosphere. When we do this in the big sims, there is NO turbulence or wind to deal with. Every day in the real world there is, and I've often wondered, as I wrestled with the 767 or A300 on final in a moderate wind, how the heck it could be done without the controls!ALL big airplanes still have at least one area where all 3 hydraulic systems are close together - the tail, specificallly the rudder assembly. The A300 that crashed at JFK probably did not crash because the tail came off, at least not in terms of direct aerodynamic effects. It probably could have been controlled in that condition, at least for a few minutes, albeit not to a real landing. (other big airplanes have been controlled in similar circumstances, at least for a time) No, the actual reason the airplane went out of control right away was that when the tail came off, so did ALL THREE hydraulic systems - in a heartbeat! So in addition to an enormously unstable airplane, they had no controls at all to deal with it, and within seconds, no engines either. We learned little from the DC-10 accident - still no hydraulic fuses or any way to isolate systems from a massive fluid loss. Ironically, the old 707 had a rudder shut off switch above the Captain's head, put there after a rudder hard over put one into that same Jamaica bay over 40 years ago...........

[Guest Ian_Riddell]

>From my 757 AOM: Automatic Trim. >The stabilizer is controlled automatically by the autopilot >or by a Mach speed trim system when the autopilots are not >engaged. The Mach speed trim system improves speed stability >by trimming the stabilizer as airspeed changes. Electric, >backup or autopilot trimming inhibits the Mach speed trim >system.Thanks, Iz.Sounds like I'll have to look into this a bit further :-)>Anyway, would this be the same as the 744 autotrim during >manual flight? I would guess so.The Mach Trim system does sound a little like the Speed Trim system, but at a different end of the airspeed scale. To be honest, I've never really understood how it works. Apparently it tries resist speed changes... so, I guess, you would have to push/pull on the stick harder than you would normally to get the aircraft to accelerate/ decelerate(?). Presumably, the aircraft would pitch up if you pushed the throttles forward too (to stop you changing speed). Sounds like this (747-400) system would make the aircraft (physically) harder to control at lower speeds... but not to the extent that the pilots complain about it... they seem to like the way it handles ;-)Anyway, thanks for the feedback (Always nice to learn something new) :-)Cheers.Ian.