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Pax Movement: Can Lead To Loss Of Control?  
User currently offlinefaro From Egypt, joined Aug 2007, 1630 posts, RR: 0
Posted (3 years 11 months 3 weeks 4 days 14 hours ago) and read 5050 times:

Hypothetically, if all pax headed to the back/front of an airliner in flight could this lead to a CG shift beyond the control authority of the stabiliser? I recall a case in WWII of a Condor taking off with a load of injured soldiers from Stalingrad who could not keep to their place; the soldiers moved en masse towards the rear during climb-out and control was lost

Faro


The chalice not my son
23 replies: All unread, jump to last
 
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 1, posted (3 years 11 months 3 weeks 4 days 13 hours ago) and read 5005 times:

IIRC, Flying Tigers lost a DC-8 this way, too: an improperly secured load of blue jeans got loose on takeoff and shifted the CG too far aft at rotation. Been a looong time since that one happened  


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinegolfradio From Canada, joined Jun 2009, 830 posts, RR: 2
Reply 2, posted (3 years 11 months 3 weeks 4 days 12 hours ago) and read 4944 times:

Supposedly a L-410 Turbolet crashed killing 19 passengers last October in Congo when passengers en masse moved to the front. A crocodile had escaped onboard. It survived the crash.

http://www.news.com.au/travel/news/crocodile-on-a-plane-kills-19/story-e6frfq80-1225942045322


User currently offlineImperialEagle From United States of America, joined Jan 2006, 2630 posts, RR: 22
Reply 3, posted (3 years 11 months 3 weeks 4 days 9 hours ago) and read 4830 times:

UA lost a Viscount in Tennesee back in the summer of '64. The aircraft was observed to be flying in an erratic manner, trailing smoke, seems like there were witnesses who claimed to have seen fire in the cabin as the aircraft got lower to the ground, then the aircraft nosed-up,stalled and crashed.

There was a theory----I don't know if it was ever proven to be correct-------that as the aircraft prepared to land (at TYB I think) a woman sprayed her hair with aerosol hairspray while a pax next to or near her lit-up a cigarette (for a quickie before landing) causing a flare-up and fire---perhaps the drapes. Anyway,it was thought that one of the cockpit crew may have opened a window as smoke filled the cabin and it created a draw towards the window. As the smoke rushed forward the panicked pax fled aft causing the a/c to stall.

I think I recall a pax wasn't willing to see how it was going to end up and they bailed-out (jumped) from an over-wing exit and fell to their death.

Might be interesting to see a copy of the accident investigation.



"If everything seems under control, you're just not going fast enough!"
User currently offlineMaverick623 From United States of America, joined Nov 2006, 5740 posts, RR: 6
Reply 4, posted (3 years 11 months 3 weeks 4 days ago) and read 4692 times:

Quoting faro (Thread starter):
Hypothetically, if all pax headed to the back/front of an airliner in flight could this lead to a CG shift beyond the control authority of the stabiliser?

In a modern airliner, all other things being equal, no. In fact, the last airline crash due to a CG problem (Air Midwest in CLT) would probably have not gone so far out of control if the stabilizer wasn't messed up due to faulty maintenance, and that was in a B1900.



"PHX is Phoenix, PDX is the other city" -777Way
User currently offlinelowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 5, posted (3 years 11 months 3 weeks 4 days ago) and read 4686 times:

If the aircraft was already loaded to one extreme end of the CG envelope, and then all the pax shifted, it might be theoretically possible. When I flew jumpers(dropped meatballs), they aircraft could get difficult to control if you had a 6 way staging in the door together. Smaller scale, I know, but it is a taste of what could happen.


Proud OOTSK member
User currently offlinejwenting From Netherlands, joined Apr 2001, 10213 posts, RR: 18
Reply 6, posted (3 years 11 months 3 weeks 4 days ago) and read 4672 times:

as said, it could happen. Depending on aircraft type, loading and speed of passenger/cargo movement it'll be easier or harder to control/prevent/recover from of course.


I wish I were flying
User currently offlinestratosphere From United States of America, joined Sep 2007, 1656 posts, RR: 5
Reply 7, posted (3 years 11 months 3 weeks 3 days 10 hours ago) and read 4447 times:

Quoting KELPkid (Reply 1):
IIRC, Flying Tigers lost a DC-8 this way, too: an improperly secured load of blue jeans got loose on takeoff and shifted the CG too far aft at rotation. Been a looong time since that one happened  


Umm I think you may be mistaken..Tigers had a few accidents but none were a DC-8 . ..I think you may be referring to the Fine Air accident in MIA in Aug 1997. Thats what happened in that accident.



NWA THE TRUE EVIL EMPIRE
User currently offlineViscount724 From Switzerland, joined Oct 2006, 26021 posts, RR: 22
Reply 8, posted (3 years 11 months 3 weeks 3 days 9 hours ago) and read 4403 times:

Quoting ImperialEagle (Reply 3):
UA lost a Viscount in Tennesee back in the summer of '64. The aircraft was observed to be flying in an erratic manner, trailing smoke, seems like there were witnesses who claimed to have seen fire in the cabin as the aircraft got lower to the ground, then the aircraft nosed-up,stalled and crashed.

There was a theory----I don't know if it was ever proven to be correct-------that as the aircraft prepared to land (at TYB I think) a woman sprayed her hair with aerosol hairspray while a pax next to or near her lit-up a cigarette (for a quickie before landing) causing a flare-up and fire---perhaps the drapes. Anyway,it was thought that one of the cockpit crew may have opened a window as smoke filled the cabin and it created a draw towards the window. As the smoke rushed forward the panicked pax fled aft causing the a/c to stall.

I think I recall a pax wasn't willing to see how it was going to end up and they bailed-out (jumped) from an over-wing exit and fell to their death.

Might be interesting to see a copy of the accident investigation.

Click on 1964 then United Airlines on the following page for the accident report.
http://ntl1.specialcollection.net/sc...dll?websearch&site=dot_aircraftacc


User currently offlineMaverick623 From United States of America, joined Nov 2006, 5740 posts, RR: 6
Reply 9, posted (3 years 11 months 3 weeks 3 days 1 hour ago) and read 4290 times:

Quoting lowrider (Reply 5):
If the aircraft was already loaded to one extreme end of the CG envelope, and then all the pax shifted, it might be theoretically possible.

It's not just possible... it WILL happen. The CG envelope is a physical property that doesn't act on a whim. What caused AirMidwest 4595 to pitch up and stall was the simple act of the landing gear retraction. That slight shift in CG made it nearly uncontrollable, combined with the elevator malfunction, it was uncontrollable.

That being said, airplanes are designed to a certain specification, based on unusual circumstances. You could arguably modestly exceed the posted limits in most airplanes, and they would still fly. The problem is if something happens: an engine fails, the wings stall, fuel leak, etc..., then the airplane may or may not be controllable.



To answer the OP's question again, though: No. A simple pax movement (again, with all other things being as they should be, within posted limits) cannot cause an airliner to go out of control.



"PHX is Phoenix, PDX is the other city" -777Way
User currently offlinelowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 10, posted (3 years 11 months 3 weeks 2 days 1 hour ago) and read 4065 times:

Quoting Maverick623 (Reply 9):
The CG envelope is a physical property that doesn't act on a whim. What caused AirMidwest 4595 to pitch up and stall was the simple act of the landing gear retraction. That slight shift in CG made it nearly uncontrollable, combined with the elevator malfunction, it was uncontrollable.

I am pretty familiar with CG calculations and how they work, having done 1 or 2 over the years. I have even taught them to a few folks. When I use the word "might" I meant that in the sense that, it may not be possible on every aircraft, or in every scenario. As to the Air Midwest accident, the incorrectly assembled elevator was the sole reason. The gear retraction would have have a net forward effect on the CG. The increasing airspeed combined with the restricted elevator travel is what caused the uncontrollable pitch up.



Proud OOTSK member
User currently offlinehal9213 From Germany, joined May 2009, 302 posts, RR: 0
Reply 11, posted (3 years 11 months 1 week 2 days 17 hours ago) and read 3426 times:

Maybe not just CG shift, but would it possible to induce an oscillation? Say, have a certain mass (some people?) move around the CG back and force in the exact frequency as the autopilot would stabilize, thus creating a ever-increasing oscillation, ultimately pitching too much up and down ? Would that be possible?

User currently offlinefaro From Egypt, joined Aug 2007, 1630 posts, RR: 0
Reply 12, posted (3 years 11 months 1 week 2 days 16 hours ago) and read 3419 times:

Quoting hal9213 (Reply 11):
Maybe not just CG shift, but would it possible to induce an oscillation? Say, have a certain mass (some people?) move around the CG back and force in the exact frequency as the autopilot would stabilize, thus creating a ever-increasing oscillation, ultimately pitching too much up and down ? Would that be possible?

I would think this is possible if the CG shift is quick enough; people sprinting to the back then rapidly sprinting forward again.

Faro



The chalice not my son
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 13, posted (3 years 11 months 1 week 2 days 16 hours ago) and read 3400 times:

Quoting faro (Reply 12):

I would think this is possible if the CG shift is quick enough; people sprinting to the back then rapidly sprinting forward again.

If you were in cruise, with the AP off, then I could see it happening.

Otherwise the AP will probably just trim it off. It only takes a heavy service cart or a few fat pax moving around to upset the CG enough to make the AP compensate for it.


User currently offline57AZ From United States of America, joined Nov 2004, 2556 posts, RR: 2
Reply 14, posted (3 years 11 months 1 week 2 days 15 hours ago) and read 3389 times:

Quoting lowrider (Reply 5):
If the aircraft was already loaded to one extreme end of the CG envelope, and then all the pax shifted, it might be theoretically possible. When I flew jumpers(dropped meatballs), they aircraft could get difficult to control if you had a 6 way staging in the door together. Smaller scale, I know, but it is a taste of what could happen.

Indeed possible. A friend of ours occasionally flys jumpers using C-54s. He mentioned that if they're at maximum capacity, the CG shift is very noticable.



"When a man runs on railroads over half of his lifetime he is fit for nothing else-and at times he don't know that."
User currently offlineflybaurlax From United States of America, joined Oct 2008, 638 posts, RR: 0
Reply 15, posted (3 years 11 months 1 week 2 days 14 hours ago) and read 3347 times:

I have seen a video of a C-2 launching off of a carrier, and it crashed shortly after takeoff. I read that it had some dense cargo that was strapped towards the front, and the restraints failed during the catapult launch. This then caused it to increasingly pitch up until stall, then splash into the water.

As stated above, yes it is possible.



Boilerup! Go Purdue!
User currently offlineVirginFlyer From New Zealand, joined Sep 2000, 4577 posts, RR: 40
Reply 16, posted (3 years 11 months 1 week 2 days 4 hours ago) and read 3246 times:

Quoting Maverick623 (Reply 4):
Quoting faro (Thread starter):
Hypothetically, if all pax headed to the back/front of an airliner in flight could this lead to a CG shift beyond the control authority of the stabiliser?

In a modern airliner, all other things being equal, no. In fact, the last airline crash due to a CG problem (Air Midwest in CLT) would probably have not gone so far out of control if the stabilizer wasn't messed up due to faulty maintenance, and that was in a B1900.

Are you certain of that? I find it very difficult to believe that there is enough leeway in the envelope to cope with several tones of payload moving tens of metres...

V/F



"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 17, posted (3 years 11 months 1 week 2 days 3 hours ago) and read 3241 times:

Quoting hal9213 (Reply 11):
Say, have a certain mass (some people?) move around the CG back and force in the exact frequency as the autopilot would stabilize, thus creating a ever-increasing oscillation, ultimately pitching too much up and down ? Would that be possible?

Only with a terribly designed autopilot. The time constants for most actuation systems are so much faster than the free response of the fuselage in pitch (or of the ability to move mass back and forth) that I suspect you'd never be able to get "ahead" of the autopilot.

Tom.


User currently offlineViscount724 From Switzerland, joined Oct 2006, 26021 posts, RR: 22
Reply 18, posted (3 years 11 months 1 week 1 day 10 hours ago) and read 3100 times:

Quoting stratosphere (Reply 7):
Tigers had a few accidents but none were a DC-8 . .

There was this one at Okinawa in 1968. Four crew killed.
http://aviation-safety.net/database/record.php?id=19700727-0


User currently offlinehal9213 From Germany, joined May 2009, 302 posts, RR: 0
Reply 19, posted (3 years 11 months 5 days 20 hours ago) and read 2921 times:

Quoting Fly2HMO (Reply 13):
Otherwise the AP will probably just trim it off.

Actually, I thought because of the AP, the oscillation may induce even further. My (laymans) thoughts were as follows:
- Some people move back of the CG initially.
- For arguments sake, lets just say, the Angle of Attack shifts 1 degree up.
- AP kicks in, has (for arguments sake) a 2 seconds delay, reacting with pitch down.
- Within this 2 secs, the people move to the front.
- Now AP has pitched down PLUS the people, so lets say now Angle of Attack is 2 degree down.
- AP kicks in, thinks it has to compensate even more, reacting with pitch up.
- Again, people now move back of CG within APs reaction time.
- Now with the stronger AP reaction plus people, Angle of attack will become (lets just say) 3 degree up.
- ...... And so on, until AoA is infinitely high ?

But...... I guess....

Quoting tdscanuck (Reply 17):
Only with a terribly designed autopilot.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 20, posted (3 years 11 months 5 days 18 hours ago) and read 2885 times:

Quoting hal9213 (Reply 19):
- Some people move back of the CG initially.
- For arguments sake, lets just say, the Angle of Attack shifts 1 degree up.

The AoA shift won't be nearly that large...moving the CG doesn't change the aircraft weight so you're not changing the total lift required. The stabilizer does have to put less downforce to hold trim...all the wing needs to do is change AoA enough to counter the changed stabilizer load.

However, your point that you could theoretically start a divergent oscillation could start from anywhere is still valid so let's proceed:

Quoting hal9213 (Reply 19):
- AP kicks in, has (for arguments sake) a 2 seconds delay, reacting with pitch down.
- Within this 2 secs, the people move to the front.

This is the kicker...no autopilot I'm aware of has a delay that long. I suspect you'd get oscillation in all kinds of cases (like glideslope tracking) with an autopilot that was that slow in pitch. What will really happen with any modern system is that the autopilot will be continually changing the stabilizer load as the people move from the back to the front...when they get to the front, the plane should still be in trim because the autopilot "kept up" with them.

Tom.


User currently offlineredflyer From United States of America, joined Feb 2005, 4376 posts, RR: 28
Reply 21, posted (3 years 11 months 5 days 14 hours ago) and read 2847 times:

Not a case where the pax movement caused a crash, but may have helped prevent one. I remember back in the '70's a L1011 suffered some type of horizontal stabilizer trim control problem locking it in a high nose-up pitch. The pilot had all the passengers move to the front of the aircraft to help compensate for it. The plane landed uneventfully.


My other home is a Piper Cherokee 180C
User currently offlineMD-90 From United States of America, joined Jan 2000, 8508 posts, RR: 12
Reply 22, posted (3 years 11 months 5 days 11 hours ago) and read 2824 times:

In the latest issue of Flying Peter Garrison analyzed the L-410 Turbolet crash last August in Bandundu, Congo (allegedly due to a mobile crocodile causing the pax to crowd up against the front of the cabin) in his Aftermath column. He goes over the probable mean aerodynamic chord of the wing, what effect that many passengers moving would have on the cg, the effect of deploying flaps, etc. Then he concludes:

Quote:
So we have an airplane with a wing loading of 32 pounds per square foot that would normally probably stall, flaps down, at about 70 to 75 knots. Its approach speed--for reasons not explained in news reports, it was approaching an "emergency strip" alongside Bandundu's main runway, and so was probably on the slow side--might have been around 85-90 knots. The maximum downforce available from the tail at this speed would be on the order of 13 pounds per suqare foot. Of this, 10 pounds would be needed to balance the passengers crowded at the front. The remaining three pounds would be needed to overcome the flap pitching moment. Nothing would be left. The nose would come down, and the airplane would begin to pick up speed. But the airplane was low and slow when the stampeded occurred; there was no sufficient altitude or time to regain control.

Was this the reason for the crash? Was there really a crocodile? A sports bag? A machete? I have no idea. But at least now I know that the scenario, however bizarre, is at least possible.


User currently offlinehal9213 From Germany, joined May 2009, 302 posts, RR: 0
Reply 23, posted (3 years 11 months 2 days 21 hours ago) and read 2650 times:

Quoting tdscanuck (Reply 20):
when they get to the front, the plane should still be in trim because the autopilot "kept up" with them.

I guess that sounds logical  
Thanks for clarifying!


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