Airbus 320 Question about dual engine failure and gliding

ca787546 · Post by **ca787546** » Sun Jun 04, 2006 2:14 pm

I have a question for any A320 drivers out there. I am puzzled, and to be honest, I can't find an answer yet, so in fear of sounding terribly ignorant about aerodynamics... here it goes:
According to the QRH procedure for dual engine failure, airbus gives a few glide ranges obtainable when flying the fixed speed of 280 kts as an optimum relight speed. These glide ratios increase with weight. I understand that completely, no questions about that. Later on, in the QRH, it also talks about flying GD speed and obtaining a certain glide ratio. Here is the part I DON'T understand, and I'd welcome any help in finding the answer. The GD ratio is 2.5nm/1000ft, while the 280 kts range with 70Tons(154Klbs) is 2.7nm/1000ft.... How is this possible, considering that GD is the best L/D speed and GD is below 280kts when flying at 70Tons?
Any help, please,
Cheers,
DNB

PS: For reference, optimum relight speed data is QRH page 1.16, and optimum speed for relight when APU bleed available (GD speed) data is on page 1.17 of the QRH (or so it is in my QRH anyway).

ca787546 · Post by **ca787546** » Sun Jun 04, 2006 4:06 pm

Help anyone???

shamrock104 · Post by **shamrock104** » Sun Jun 04, 2006 7:20 pm

Try posting on http://www.pprune.org

ahramin · Post by **ahramin** » Sun Jun 04, 2006 8:05 pm

Well, i have never sat in a 320 so take the following with that in mind.

First draw your A320 in a glide. Draw lift, weight, thrust, and drag. Thrust is the component of weight which is opposite to drag. So you can see that the heavier the plane, the more the thrust. At the same time, weight does not affect drag. So the heavier you are, the farther you can glide.

But wait, i hear you cry, if weight increases, then lift must also be increased to compensate. And more lift equals more drag. And you would be correct. The heavier you are, the faster your best glide speed. Put on enough weight and you have to fly so fast that you reach the point of diminishing returns as your parasitic drag wipes out the gains of your high wing loading.

For each weight there is only one best glide speed, and only one weight will give you your best gliding distance. However the difference in gliding distances from lowest weight to highest weight is usually not very significant. An 8% change seems like a lot to me. The reason you see gliders loading up with water is not to get a better gliding angle so much as to get that gliding angle at a higher speed.

As far as why the published glide speed has a lesser glide ratio than the relight speed on your A320 maybe it is because they are for different weights or (wild ass guess) maybe it is a compromise between best glide and best endurance in order to give you time to sort out your little problem so that you won't end up needing best glide distance.

Hope that helped.

ca787546 · Post by **ca787546** » Sun Jun 04, 2006 10:43 pm

Thanks Ahramin, that did help out, but still doesn't really explain why 280kts would give me such a high glide ratio. By definition, airbus says that the green dot speed (which is constantly computed and displayed on the PFD for different weights, altitude, temperature, etc etc based on sensors such as TAT, IAS, altitude, AOA, etc etc) is the best Lift/Drag speed. From the little I know, the best L/D should give me the best glide ratio, and if I remember, a difference in weight will not change this glide ratio, it'll simply change the speed and V/S, thus more weight equals higher speed and higher vertical speed, but same distance covered by altitude lost. In the 320, the best L/D never reaches 280 knots (not even at MCTOW). To the best of my knowledge, the more you get away from the best L/D speed, the worse your glide ratio becomes, that is why I don't understand that 8% difference in favour of 280 kts over best L/D. Anyway, I'm still a bit puzzled by this.
Thanks anyway,
Cheers,
DNB

wannabatp · Post by **wannabatp** » Mon Jun 05, 2006 5:29 am

Try and get in touch with Piche and Yaeger from Transat. They might be able to tell you a thing or 2 about gliding an Airbus.

ahramin · Post by **ahramin** » Mon Jun 05, 2006 10:18 am

Like i said i am out of my depth with your A320, but yes, best L/D will always give you the best distance in calm air. At the speeds you guys would be gliding at it would take a hell of a wind to make any difference.

Maybe a misprint?

As for Piche and Yeager, i think they were too busy crapping themselves.

TOGA Descent · Post by **TOGA Descent** » Mon Jun 05, 2006 6:19 pm

ca787546 wrote:I have a question for any A320 drivers out there. I am puzzled, and to be honest, I can't find an answer yet, so in fear of sounding terribly ignorant about aerodynamics... here it goes:
According to the QRH procedure for dual engine failure, airbus gives a few glide ranges obtainable when flying the fixed speed of 280 kts as an optimum relight speed. These glide ratios increase with weight. I understand that completely, no questions about that. Later on, in the QRH, it also talks about flying GD speed and obtaining a certain glide ratio. Here is the part I DON'T understand, and I'd welcome any help in finding the answer. The GD ratio is 2.5nm/1000ft, while the 280 kts range with 70Tons(154Klbs) is 2.7nm/1000ft.... How is this possible, considering that GD is the best L/D speed and GD is below 280kts when flying at 70Tons?
Any help, please,
Cheers,
DNB

PS: For reference, optimum relight speed data is QRH page 1.16, and optimum speed for relight when APU bleed available (GD speed) data is on page 1.17 of the QRH (or so it is in my QRH anyway).

Interesting question, and I don't have the answer off the top of my head. I'm going to work in about an hour, research it during the flight, and get back to you tomorrrow.

ca787546 · Post by **ca787546** » Mon Jun 05, 2006 6:32 pm

Hi TOGA descent... I hope you don't try it during the descent, because you won't get any real data unless you decide to actually turn off the engines and stop them from physically turning (they may produce residual thrust or drag). Anyhow, thanks for the try. I am have recurrent training on the sim in 10 days, I'll ask about it then and hopefully get an answer.
Cheers,
DNB

airbus_rookie · Post by **airbus_rookie** » Mon Jun 05, 2006 6:38 pm

DNB,

I think you are confusing two different issues.

QRH 1.16 initially has you assume an Optimum relight speed, to attempt a windmilling start, until pneumatic pressure becomes avail from APU. It goes on to say that if this doesn't work, go to green dot speed.

As you have correctly indicated Green dot is optimum glide speed.

Or am I missing something?

Regards.

rsandor · Post by **rsandor** » Mon Jun 05, 2006 7:17 pm

airbus_rookie wrote:DNB,

I think you are confusing two different issues.

QRH 1.16 initially has you assume an Optimum relight speed, to attempt a windmilling start, until pneumatic pressure becomes avail from APU. It goes on to say that if this doesn't work, go to green dot speed.

As you have correctly indicated Green dot is optimum glide speed.

Or am I missing something?

Regards.

Here is the manual page:

I think the original question is: How is it possible that GrnDot speed, (which should give the best glide distance) can give LESS range than the 280kts relight speed?

ahramin · Post by **ahramin** » Mon Jun 05, 2006 7:57 pm

So what is the glide ratio from the manual at green dot speed?

ahramin · Post by **ahramin** » Mon Jun 05, 2006 8:01 pm

LAND ASAP

Is this a 2 engine out checklist? LAND ASAP would seem to indicate to me that you have some choice in the matter, i.e. one engine still going.

Or is it just telling you to land asap once you have a relight?

Or maybe it is reminding you that you are going to land asap

rsandor · Post by **rsandor** » Mon Jun 05, 2006 8:06 pm

ahramin wrote:So what is the glide ratio from the manual at green dot speed?

Read the .jpg above: It says that at GreenDot speed, the a/c can fly aprox 2.5nm per 1000 feet. (best l/d ratio) But above that, it shows 280kts getting 2.7nm.

I think it may just be the wording, and that the given 2,5nm for GreenDot is an example (@60,000kg), and not an absolute. I think maybe it too would increase with weight, no?

ca787546 · Post by **ca787546** » Mon Jun 05, 2006 8:07 pm

OK, here is the matter at hand!!! To make things clear, for airbus rookie and everyone! Relight speed (280kts, higher speed than best L/D which is green dot speed) with 70tons gives me 2.7nm/1000feet, while Green dot speed gives me 2.5nm/1000 feet.....
My question is... if you're flying faster than best Lift/Drag speed, why are you getting a better glide ratio?????
Ahramim, the land ASAP only shows because that's what the ECAM would say. The ECAM assists the crew in making this decision by indicating LAND ASAP
either in amber or red:
. If an abnormal procedure causes LAND ASAP to appear in amber on the
ECAM, the crew should consider the seriousness of the situation and the
selection of a suitable airport
. If an emergency procedure causes LAND ASAP to appear in red on the
ECAM, the crew should land at the nearest suitable airport.
Cheers,
DNB

ahramin · Post by **ahramin** » Mon Jun 05, 2006 8:15 pm

Whoops, missed that bit. No idea why green dot would give you less of a glide ratio than any other speed.

Note on the relight speed though. The reason the glide ratio changes so dramatically is not because your best glide ratio is so different, it is because you are flying at a much higher speed than your best l/d speed. You should still be able to get a glide ratio very close to 2.7 (or 2.5) at any weight, but not at that speed.

ca787546 · Post by **ca787546** » Mon Jun 05, 2006 8:19 pm

I hope all know that best L/D speed and green dot speed are exactly the SAME thing in airbus.

Also, 280 kts is higher than best L/D(green dot) speed, thus, you should have a lower glide ratio at 280 kts. Now, why is airbus telling me that if I have 70tons and fly 280kts I'm getting a better glide ratio? That is why I want to know, because to me, it DOESN'T make sense.
Regards,
DNB

xsbank · Post by **xsbank** » Mon Jun 05, 2006 10:49 pm

It seems to me that the higher speed will give you a better RATIO, but you will be approaching the ground faster at the higher speed, yet if you fly at 'green dot' you will get a worse RATIO but will stay in the air longer? I'm struggling with this...

By the way, if you have a total engine failure and do not have a generated speed to follow, you are very close if you choose V2; if you are high you would have to apply a correction; as well as a correction to your standby airspeed indicator. This will do until you can get a more accurate number from the QRH charts.

TOGA Descent · Post by **TOGA Descent** » Tue Jun 06, 2006 5:25 am

Oops, see below

ca787546 · Post by **ca787546** » Tue Jun 06, 2006 5:28 am

If you have a dual engine failure, you will still have a calculated green dot on your PFD. Since green dot is best L/D speed (accurate for actual conditions of weight, altitude, temperature, etc) it SHOULD give you the longest range (best glide ratio). What I don't understand is WHY the QRH tells you that at 70tons flying 280 kts you get a better glide ratio (longer range). It just doesn't make any sense whatsoever to me.
Regads,
DNB

TOGA Descent · Post by **TOGA Descent** » Tue Jun 06, 2006 5:33 am

I think I have the answer to your question.

Boeing includes more detailed data in their QRH than Airbus provides in theirs. In fact, Airbus will tell you that a lot of the data in their QRH is “Assumptive”.

What that means is – in a dual engine failure scenario – that the “Assumption” is that the crew will change the configuration of the aircraft to reach a prescribed Engine Relight speed, and complete this entire procedure, before moving to the next page in the QRH (Keep Reading).

Following that part of the QRH, it “Assumes” that the crew you will now change the configuration, again - to obtain GD speed - and fly EACH green dot speed until touch down. (Green Dot is, either and all of, “GD” with flaps at “Zero”, “S” speed with Flats at “1”, “F” speed with flaps greater than “1” and “Vapp” during final approach.)

Here’s where the answer to the question finally arrives.

The glide ratio at GD does not calculate a decent to ground level as it does with the Engine Relight Speed. At GD, it calculates the glide distance to approximately 3000 feet AGL.

Again, it “Assumes” that you will fly an approach procedure, (Don’t know how they came up with that) and select Flaps and Landing Gear - for a landing - at an appropriate time during the glide, where the Engine Relight Speed does not make this assumption.

This 3000 foot “Assumed” floor, and the changing Green Dot Speeds described above, leads to a somewhat reasonable answer to your question.

Please don’t hold me to this when both engines fail. I’m just repeating what I was told.

Obviously, I’m curious enough to be open for a discussion about this one

TOGA Descent · Post by **TOGA Descent** » Tue Jun 06, 2006 5:40 am

xsbank wrote:By the way, if you have a total engine failure and do not have a generated speed to follow, you are very close if you choose V2; if you are high you would have to apply a correction; as well as a correction to your standby airspeed indicator. This will do until you can get a more accurate number from the QRH charts.

Since we are discussing the Airbus specifically, the APU or RAT will supply power, at least, to the Captains PFD where you can read GD speed.

However, in the event of a complete power loss, you'll be much closer to Green Dot, if you look at your load sheet. Then, take 150% of your current estimated weight - in tonnes - as a speed (ie: 70 T = 105 kts) then add 115 Kts for a total of 220 KIAS. This will normally be within 5 knots of your CLEAN GD speed. Then, you can read that speed from the Stand-by ASI.

V2 would be around 160 knots, and is calculated in a specific flap configuration (Not Clean) so it would not even be close to the correct number.

TOGA Descent · Post by **TOGA Descent** » Tue Jun 06, 2006 6:45 am

ca787546 wrote:Ahramim, the land ASAP only shows because that's what the ECAM would say. The ECAM assists the crew in making this decision by indicating LAND ASAP either in amber or red:
. If an abnormal procedure causes LAND ASAP to appear in amber on the
ECAM, the crew should consider the seriousness of the situation and the
selection of a suitable airport
. If an emergency procedure causes LAND ASAP to appear in red on the
ECAM, the crew should land at the nearest suitable airport.
Cheers,
DNB

“LAND ASAP”. As CA787546 indicated, ECAM displays this either in red or amber. One opinion out there on the line is that, if it’s in red, you need to land as soon as possible…nearest suitable airport (however that’s defined???).

If the ECAM displays “LAND ASAP” in amber, the captain has a little more discretion in the decision of when and where to land.

Unfortunately, many FOM’s (Flight Operations Manuals) have different ideas. In fact, some do not make any differentiation for the Airbus ECAM red and amber displays.

The first sentence in one Major US Airlines FOM page states, “...The FAA does not define suitable airport in this context...” [the context being…an emergency or non-normal]. The text goes on to say, “...The captain and dispatcher determine suitability based on all factors relevant to the situation...”.

That FOM lists eight factors that you (the Captain) and the dispatcher should consider in making this decision.

Typically, in international operations, you have to use your own judgment.

Personally, I define a "suitable" airport as follows:

If the ECAM is Red, I land anywhere with an adequate runway, and appropriate Crash / Fire Equipment.

If it’s in Amber, I think the above - obviously - and about other “convenience” issues (Customs and Immigration, Hotels, Connecting Flights, Base of Operations, etc) that I would not consider with a Red indication.

xsbank · Post by **xsbank** » Tue Jun 06, 2006 9:50 am

My tip about V2 was a generic reply and not an Airbus one as I know dick about them except that they have no respect for the crew, constantly calling them "retard(s)!"

My experience is GEX and it likes its crew.

sidestick stirrer · Post by **sidestick stirrer** » Tue Jun 06, 2006 10:27 am

If the ECAM is Red, I land anywhere with an adequate runway, and appropriate Crash / Fire Equipment.

If it’s in Amber, I think the above - obviously - and about other “convenience” issues (Customs and Immigration, Hotels, Connecting Flights, Base of Operations, etc) that I would not consider with a Red indication.[/quote]

I simplify this somewhat for my students/candidates:
LANDASAP in red means the airplane's ECAM logic has determined that,"Hey, I'm dying here, get me on the ground!" This would occur when following the ECAM actions and/or appropriate checklists, the situation has not stabilized, it is still dynamic, and deteriorating.
LANDASAP in amber indicates that the situation has stabilized, but the airplane is not as capable as it was, and landing short of the original destination should be considered if those lost capabilities would be required along the way or upon approach/landing there.
For example, with a fire warning, LANDASAP immediately comes up in red, once the fire warning is extinguished, it is still displayed, but changes colour to amber.

Airbus 320 Question about dual engine failure and gliding

Airbus 320 Question about dual engine failure and gliding

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Re: Airbus 320 Question about dual engine failure and glidin

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