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Anyone Practice Phugoid?

Sun Aug 19, 2007 7:47 am

I was playing in the simulator (C-5) yesterday after our session and decided to try my hand at some phugoid practice. Basically, we shut off three of the four hydraulic systems (#1, #2, and #3), rendering the elevators and rudders useless and leaving me with just the right aileron and a few flight spoilers (and, of course, engine power). We entered a phugoid with extreme pitch oscillations which I was eventually able to dampen out. I got headed for a runway and set up a pretty stabilized approach at about 180 KTS (clean configuration). I put it down right on centerline in the touchdown zone but my VVI on landing was around 900 FPM and I got the red screen of death. The second time I put in full power too early and ballooned it back into the air. The third time, I bounced and again the plane wanted to fly. On my fourth attempt, we finally lived. I made it a much shallower approach with partial flaps with about 300fpm rate of descent (as best I could, anyway). This time I landed short and took out some approach lights but it was survivable.

Obviously, losing most of your hydraulics is very bad news on a big airplane (there's no "manual reversion" w/ the C-5). This caused the loss of a C-5 at Siagon in the '70s and of course United 232. Have any of you ever tried this scenerio in the sim? How did it work out for you?
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RE: Anyone Practice Phugoid?

Wed Aug 22, 2007 1:01 am

Yes, I did try it when I first checked out on the DC-10. That airplane has a long cycle phugoid as was experienced on UAL 232 and my approach turned out about the same even though I had the advantage of not having structural damage and having thought about the scenario in advance. Some work has been done since that crash to use the autopilot to drive the engines to control the attitude and path but this has never been incorporated into the trijets.

In fact, the DC10 has a phugoid response in it's basic aerodynamics. Generally, the autopilot will overcome this during the climbs and descents. However, it can be seen when the autopilot is attempting to maintain a specific IAS during high altitude climb or idle descent. The airplane 'hunts' to maintain speed. That is, it pitches up and down over a couple of minutes getting a little too slow, then a little too fast. The cure is to either turn off the autopilot or place it into the vertical speed mode. This damps out the tendency to wander in pitch.

Of course, with a loss of hydraulics, there is no autopilot and no flight controls. All that is left is the natural tendency to remain in trim speed subject to the inherent stability of the design. The phugoid is a degree of instability but because it is a long cycle, not divergent, and normally can be easily damped out through direct control input or autopilot, it is acceptable. Because a loss of all hydraulics is such a remote possibility, there is little point in practice of a maneuver with so little application, low probability of mastering, and a low probability of a successful outcome.
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RE: Anyone Practice Phugoid?

Wed Aug 22, 2007 2:15 am

Quoting 113312 (Reply 1):
Because a loss of all hydraulics is such a remote possibility, there is little point in practice of a maneuver with so little application, low probability of mastering, and a low probability of a successful outcome.

Yes, I know that after the United incident, NASA did some tests on systems controlling an aircraft (MD11 and F15) through to landing, using just thrust.


I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!

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