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dubfan
19th Jan 2008, 19:15
Hi Folks,
Not interested in starting any form of a thread on the accident itself - more a generic question based on technical consequences of lowering/retracting flaps and gear to stretch a glide.

Am hearing a lot in the media, and indeed reading some opinions on the matter here on pprune, about the effects of reducing (or even increasing)flap on short final to increase the glide distance. One genius expert on TV even suggested raising the gear.

I remember back to my PPL/CPL days and doing the practice forced landings and the glide approaches. We were told never, ever commit gear down till we were sure we would make the field and never ever ever drop full flap till we were positive we would. Finally, once configured, never ever ever ever deselect them.

Technical opinions please based on lift/drag. (Disregard ability of crew to even think along these lines)

DF

BOAC
19th Jan 2008, 19:55
I don't think I would have had time to carry out the necessary performance iterations from 600', so if I were to reduce flap in that situation it would be a 'gut feeling' move only. Maybe they did? Certainly despite what you may read from 'the experts' on the other (amazing!) forum, reducing a landing flap setting will not reduce the lift much but will reduce the drag, therefore it will possibly benefit. However, see Rule 1:)

Gear MUST be left down to absorb the energy of impact in that situation. Raising it MIGHT allow a stretch of the glide, but airliners' drag normally increases as the gear goes up and down due to various doors opening and shutting, so it would need to be done at sufficient height to make a gain, and 600 is not that!

Having said all that, the advice you have been given is spot on:ok:

aamandani
19th Jan 2008, 20:35
Variations on Aircraft Performance


Configuration Change:
DirtyParasite Drag is increased
Maximum velocity is decreased (structural limitations)
L/Dmaxairspeed decreases (not a) (configuration dependent)
Tr increases (to provide thrust to counter increased drag)
Taremains the same
Reduced climb angle
Reduced rate of climb
Fuel flow increases (proportionate to thrust increase)
Absolute ceiling decreases (higher Tr/Taratio)
Maximum Endurance is decreased
Maximum Range is decreased
CleanOpposite of all above

IN THIS CASE IT WOULD BE BETTER IF HE RAISED THE FLAPS BACK UP AND AND THE GEAR WHICH WOULD MOST PROBABLY STRETCH THE GLIDE AS THE L/D MAX WOULD BE INCREASED AND MADE A FLAT BELLY LANDING BY SHUTTING OF THE FUEL FLOW AND EVERYTHING SO THAT FIRE WAS NOT AN ISSUE..BUT HEY HE WAS 600 FEET IN 777 NO TIME FOR ANYTHNG..WE SHOULD NOT CRITISIZE THEM AS WE WOULD DO THE SAME THING IF WE WERE IN THEIR SITUATION..ALL IN ALL THEY ALL DID A GOOD JOB..SAFE FLYING TO EVRYONE OUT THR..

galaxy flyer
19th Jan 2008, 21:10
The KDOV C-5 crash two years ago, the pilots were in a similar situation, in the sense they were running out of power. Yes, yes, they got there by a very different route. BUT, the point is that at that altitude and speed, reducing flap could very easily induce a stall and loss of control. The C-5 guys sucked up the flaps at about 500 feet and promptly stalled into the ground.

I think the BA crew did it exactly right, fly the plane and take the short landing UNDER control. Excellent job that NO one is trained for.

GF

judge.oversteer
19th Jan 2008, 21:53
Gear.
Do not retract the gear! Retraction will cause more drag by opening the gear doors.
The evidence has shown the gear absorbed most of the energy.
Flaps.
If you could, retract flaps to to min L/D ratio for approach i.e. F20/F10.
IMHO.
I dont think you'd have time to think about it at 400 ft.
Cheers
JO

dubfan
19th Jan 2008, 22:13
First off, I am in no way looking to judge what was or wasnt done by the crew. Its a given that they all, both front and back did an outstanding job - please see initial paragraph ofopening post. Tx

I am more interested in some of the suggestions being made by the armchair experts in suggesting that playing with the landing configuration, will give the crew a few valuable extra yards - full stop.

I personally dont think so and in fact am convinced that any tinkering with the config at that altitude would in fact have reduced further the touch down point.

All above responses are appreciated and most comfirm my original suspicions.

BOAC - excellent point on both the value of having the gear down if only to absorbe considerable energy at impace - valuable energy the crew n pax didnt have to. Also the transition from gear dn to gear up must surely dramatically alter the drag, not to mention the handling?.

Its the altering the flaps that most interests me. Loss of thrust is somewhat replaced by the forward moment of weight as the a/c is decending on a glide - not inconsiderable I presume in a 777. So this should give some additional energy worth managing. If full flap was already out, then as has been already been said, the only benifit the crew has from that is a lower stall speed, the penality being more drag. To reduce from full flap (often refered to as drag flap) would cause an initial tendancy to sink unless the nose is raised, which would stall the a/c if the original speed was at the lower stall speed. I can only see benifit in extending from some intermediate flap setting to landing flap at a late stage to perhaps allow a brief but valuable raising of the nose for the last x amount of feet.

I guess my question is, again, is there any benifit at all??

TX

Contacttower
19th Jan 2008, 22:25
OK this probably isn't really relevant to the 777 but if one is doing a PFL in a light aircraft and deploy full flap early (ie before you know you can 'make it') the glide angle becomes steeper and the plane sinks faster for a given speed, because in general the last stage of flap adds a lot drag and not much lift (if any) or decrease in stall speed. If one subsquently raises the last stage of flap (apon seening the approach angle shallow) the aircraft starts to glide better again. I don't know but I'd have thought that raising the flaps a bit would have increased the gliding distance in the case of the 777.

FE Hoppy
20th Jan 2008, 00:41
I don't know the 777 but i know a bit about the FBW system as a similar system from the same company is used on the E-jets I teach. In our FBW we have AOA limiting which means if you pull back to the stop you will be limited to MAX AOA for the current config. i.e. Cl max. If the 777 is the same and they pulled back to the shaker after the initial power loss the aircraft would be at Cl max. If they then retracted flap the FBW system would pitch the aircraft to maintain the max AOA for the new configuration. While I am aware that Cl max is not best L/D I would hazard a guess that thats what they flew.

It's all theory but the next time I have 5 minutes spare in the sim I'm going to see what effect retracting flap when AOA limiting is active has.

WeekendFlyer
20th Jan 2008, 01:04
I think 737Jock has it right. Although in theory retracting the flaps by one stage would increase the max L/D and lengthen the glide slightly, it would almost certainly increase the stall speed and possibly reduce the amount of CLMax available, which for a crash landing would be a very bad thing, given that you are trying to hit with the lowest speed and ROD possible.

AIUI modern FBW jets will not permit the stall to be entered, instead holding the AOA just below the stall. If the aircraft was at or near AOAmax when the flaps retracted, I would expect the FCS to command a pitch down to reduce the AOA, which could actually worsen the glidepath.

IMHO the crew did the right thing leaving the configuration as it was and concentrating on flightpath control and trying to sort out the engine problems.

FE Hoppy
20th Jan 2008, 01:18
AIUI modern FBW jets will not permit the stall to be entered, instead holding the AOA just below the stall. If the aircraft was at or near AOAmax when the flaps retracted, I would expect the FCS to command a pitch down to reduce the AOA, which could actually worsen the glidepath.


Pitch down or up?

My guess would be pitch up as retracting the flaps when maintaining pitch would reduce aoa.

Either way I don't think it would be a good idea to move em.

WeekendFlyer
20th Jan 2008, 01:37
Hoppy, fair point! But then again perhaps the AOA reducing with the flaps retracting would co-incide with the change in AOA limit, leaving the pitch angle essentially the same. Any 777 FCS experts out there care to comment?

Joetom
20th Jan 2008, 02:44
1. Lifting the gear may produce a longer glide angle, but drag will increase to start with, gear is powered by centre hyds, two big air driven pumps and two little elec ones I think, who knows how these systems were going to be working at that time, result could have been just more drag and some of the gear not locked down or half way up and pitch changes going on and poss slow operation of flt controls due big hyd use on gear.

2. Changing flap setting may produce a longer glide angle, but due hyd issues and the need to change pitch and so close to the ground appears a close call either way.

With both of the above and headwind/tailwind to take into account, the Aircraft would of had a best/max glide angle config, appears the crew got it right, any shorter did'nt look nice and much further could of been a big problem going on the paved surface at hi speed, I guess there is a perfect point, config, speed, pitch and roll for an undershoot, looks like they were very close on all counts, well done !!!

Wornout Rubber
20th Jan 2008, 07:51
I do not know what Flap setting the 777 used, but Flap 25 is a normal flap setting for landing on BA 777s.

Let`s look at some figures:

At a weight of 200T (a typical landing weight), Gear down on a 3 degree glideslope:

Flap 25.........Vref 142 kts........pitch attitude 0.5 degrees....
N1 power..........50.7 %

Flaps 20.......Vref 147 kts............pitch attitude 1.5 degrees....
N1 power...........40.2%

These figures are for a 777-ER Trent 892, using an extra 5 kts of speed (Using Vref 25/20 +10, as opposed to the normal Vref25/20 +5).

The figures are from the Boeing 777 QRH P1.20.12.

What is interesting to note is the difference of only 5 kts between Vref 20 and Vref 25. Also at Flap 20, there is a substantial reduction in drag and hence thrust required. ( N1 thrust 40.2% for F20 as opposed to 50.7% for F25).

The crew would have little or no time to consider these factors with only seconds available to handle a particularly nasty situation. What they did in the time available was commendable. They all walked away, which is the most important thing of all.

Dani
20th Jan 2008, 08:13
You never try to do a belly landing (i.e. landing without landing gear down) in an airliner. Gear struts are attached to hardened point on the most strengthened airframe structure.

So if you're in doubt if you can make it to the RWY or not, DON'T retract the gear.

When you crash land with the gear down, part of your impact energy is taken away by the gear, so less force against the rest of the airframe.

If you crash land without the gear, all impact energy goes into the fuselage, this cracks and brakes up, as we have seen so many time.

Dani

aussiepax
20th Jan 2008, 09:06
Amateur question ; If there was a software / FADEC problem, would it be any different with which brand of engine was being used : RR Vs GE , or impossible to say ?

Bunk-Rest
21st Jan 2008, 09:11
Impossibly stupid to speculate I would say.
I don't believe any airline training includes crash landing techniques.
Perhaps you gentlemen could offer your services.

The Swinging Monkey
21st Jan 2008, 10:37
Gentlemen,

As a mere ex RAF Winchman, may I ask a question that has been asked of myself several times since this incident?

We are always told (as SLF) that we must switch off all electronic items in the cabin prior to take off and landing ie Phones, Laptops etc. And we all know that just prior to landing, all the numpties get their phones out and switch them back on!!

1. Is it possible that some kind of interference from a phone(s) could be the 'cause' ?
2. What aircraft systems DO mobile phones interfere with?

An outstanding job by all of the crew last week, well done to you all.

Kind regards
The Swinging Monkey

Beta Light
21st Jan 2008, 10:43
Yes I am B777 rated ( not current ). At 600' in landing conf. the crew did the right thing in my humble opinion.....drag from gear doors, increased speed ( that needs an alt. trade off ) it is best to live with what you've got.

What was most impresive to me was the dissipline to honor the speed, the biggest temptation will be to stretch the glide, especially so close to the runway. This will lead to stall, stick pusher etc. and uncontrolled crash.

Hat off to the handeling pilot :D

P.S. Don't believe everything you hear / see in the media regarding config change etc.

ITCZ
21st Jan 2008, 11:28
We are always told (as SLF) that we must switch off all electronic items in the cabin prior to take off and landing ie Phones, Laptops etc. And we all know that just prior to landing, all the numpties get their phones out and switch them back on!!

1. Is it possible that some kind of interference from a phone(s) could be the 'cause' ?
2. What aircraft systems DO mobile phones interfere with?

Interesting question, this one provided some entertainment as I browsed a non-professional aviation forum, which contained an amusing hypothesis: The UK PM was in the terminal at the time, therefore some mobile phone band jamming equipment favoured by his protective detail was in operation!

My understanding of the 777 FADEC for both engine types is a dual-channel Engine Electronic Control unit for each motor. To have the EECs shut down both motors simultaneously would (I imagine) require a malfunction of all four EEC control channels. I am only relying on my knowledge of FADEC function in BR715, a different engine from a different manufacturer, but with a similar EEC.

Control of the EEC requires, among other things, digital signalling from the throttles (linear variable differential transformers) that provide a throttle/thrust lever resolver angle to the EEC. Other inputs to the EEC for aircraft and engine configuration, pressure and temp sensing at various engine stages, etc, are also inputs to the EEC.

I'm not an aircraft engineer nor an electronics/RF specialist, but I would be surprised if RF interference from the sources you mentioned would be of any nuisance to autothrottle and FADEC operation.

RF interference is more likely to be a problem for those systems that use/sense RF transmissions. Comms, nav equipment, etc. Example: I regularly hear the 'beep-beep, beep-beep' of GSM phones logging into the nearest mobile phone tower on taxi in, despite instructions to remain switched off until inside the terminal! Also, GPS satellite signals are several factors weaker than normal background radiation, certainly less than the gain of mobile phone Tx.

I'd be surprised if this were due to mobile phone Tx, or the PM's jammers!

ITCZ
21st Jan 2008, 11:59
Back on the subject of flap selection- is there a benefit? Some interesting points come up...

What is interesting to note is the difference of only 5 kts between Vref 20 and Vref 25. Also at Flap 20, there is a substantial reduction in drag and hence thrust required. ( N1 thrust 40.2% for F20 as opposed to 50.7% for F25).

Two good points. First is, Vref increases 1kt for each 1 deg of landing flap 'you dont have' which is a rule of thumb in some older jets and turboprops.

Second, the big difference in thrust required to 'balance' the drag in a transport category aircraft when flaps are set to landing range. Quite different to the wing we were flying when we were doing PFLs in our ab initio. Again it is through the mists of time, but during our type ratings we all would have seen the very high sink rates and huge amount of thrust required to recover from a stall or incipient stall in the landing configuration, throttles closed.

Another general characteristic of flaps on transports can be found by going back to another rule of thumb from an earlier generation: flap for circling. Take a look at your FCOM or company training manual and note the recommended flap (and speed) for a circling approach, or bad weather circuit. It also probably corresponds to the max flap for T/O setting. That would be the flap setting at which, very roughly, proportionally 'more lift added than drag.'

But its all academic. If you experienced a repeat of this situation, selecting a lower flap setting would not help you, even if it was a 'better' glide range flap, whatever that might be. For you would be in the same predicament as after a decreasing performance windshear event - you would be too slow to take advantage of the 'better' glide flap. You would first need to push the nose down to accelerate. And in a 50 tonne or 250 tonne transport, that is a massive change of momentum, that is going to use up a lot of altitude, of which you have precious little.

fly the plane and take the short landing UNDER CONTROL

:D

Dead right. Lose all thrust in that situation, not even Bob Hoover will make the runway. Forget the runway. Fly it to the ground under control.

Butter side-up
30th Mar 2008, 03:26
To continue the original posting...

Does anyone know what the relationship is between the stall speed of an aircraft (in any given configuration) and it's best glide speed in that configuration?

Assuming that they were at approach speed (Vref +5, say), and knowing that Vref is something like 1.23 VS for a given configuration I am interested to know whether in theory the aircraft should have been accelerated by pitching down to extend the glide.

Interesting though it is, I do not want to go through the discussions about configuration changes.

Thanks for any input.