High altitude stall

[Kimmar]

I should be taking my own advice and not reading the media during these next couple of weeks after Air France, but this one got by my radar, and now I'm wondering if this is actually a problem?

From what I learned at flight school even if a plane stalls it's still capable of righting itself, and gliding to get airspeed, no matter how big the plane. The stall becomes a problem when you are close to the ground.

From what I read a pilot is speculating that the airplane stalled when the pilot slowed too much to counter for turbulence. He stated that once it was stalled at high altitude that it would have been un-recoverable and he wasn't surprised if the plane broke up in mid-air, (or something to that effect.) Am I wrong, or is this not right? (This is actually my worst fear confirmed, if it were true...)

It seems like this would be the industries best kept secret if it were true, as all the ground school training and research I've done suggests otherwise...

I'm pretty sure any airline pilots flying jets would have stall experience, since it's something they teach you when you fly your first Cessna...

[scottr0829]

To add to Kimmar's question - what about a stall at high altitude while still heavy? They were only 3 hours into an 11 hour flight so they were still quite heavy from the weight of the fuel.

I have some questions about FBW flying but I will not post them here.

Thanks Captain Hutch!

:cool:

[Barb-SAN]

They are discussing this on PPRuNe too...someone linked this article: http://economictimes.indiatimes.com/News/PoliticsNation/Crashed-Air-France-jet-flying-at-wrong-speed-Report/articleshow/4616809.cms
....In a brief report on its website, the newspaper said the plane's manufacturer Airbus was preparing to send a warning to the operators of the hundreds of A330 jets in the world with new advice on flying in storms....
Once again we see that as tragic as it is to have an accident, the aviation industry is learning from it and making changes to improve safety for future flights.

[Barb-SAN]

To add to Kimmar's question - what about a stall at high altitude while still heavy? They were only 3 hours into an 11 hour flight so they were still quite heavy from the weight of the fuel.:cool:
A related question...and I'll ask it as a "theoretical question", since we don't know yet all the particulars of this accident. What effects could be expected if the plane was caught in an updraft from a thunderstorm...taking it to a still higher altitude? And of course, I'm thinking that the chance of getting caught in a strong updraft or downdraft is one reason why pilots are supposed to avoid, deviate, turn back rather than fly through thunderstorms.

What about flying OVER them? Are you allowed to fly over them if there is a sufficiently large distance between the plane and the top of a CB?

[MathFox]

If you can keep enough distance you can fly over the storm; in the case of CB tops at FL450 it is not practical for most airliners.

If you get in an updraft, pitch the nose down, apply spoilers to control your speed and try to steer out of it. (Heard from a glider pilot who found himself in a developing thunderstorm.) Recovering from a downdraft requires engine power.

[Barb-SAN]

Actually, my question wasn't so much about recovering from an updraft. Following Scott's thinking about a high altitude stall.. theoretically, could an updraft from a thunderstorm push an airplane higher than it's certified to fly? I know, imagination in overdrive.....and I'm still confident that pilots on my flights will avoid thunderstorms! :tongue:. Presumably the pilots WOULD notice the altitude increase if they were in an updraft, and do something immediately to counteract it.

[MathFox]

The certification limit is something that means a lot of paperwork for the pilot when he exceeds it. Planes are designed with some margins around the certified limits... Flying within limits is safe, flying outside the limits can be safe, but should be avoided.

At higher altitude the margin between "stall speed" and maximum speed is small; that is something the flight crew has to consider when entering and area of turbulence. It is good to have a bit more room for speed variations and opt to fly a few 1000ft lower when there's turbulence predicted. I have no doubt that the Air France pilots took that into account in their flight planning.

[waiting for hard facts]

[Captain Hutch]

Okay you guys, I'll bite. You weren't supposed to be reading the media reports, remember? :shakehead (well, that's what we're here for!)

Basically there are two limiting airspeeds at cruise altitude, one a minimum and one a maximum. The minimum speed keeps you safely above the stall speed and the maximum speed is called buffet margin. As the plane flies at speeds close to the high speed buffet margin the aerodynamics of the flight surfaces become less efficient because some areas of air flow are approaching the mach, or the speed of sound, and flight characteristics are somewhat unpredictable. Airliners cruise at speeds between these two limits. The higher and heavier the airplane is, the smaller the range between these two limits. Turbulence Penetration Speed, or the recommended speed for flying through turbulence, is a specific number that will always be inside of this range and is specific to the type of aircraft.

In my 737's we have a handy dandy little chart that has the speeds, weights and altitudes listed, or they're also available in the flight management computer. Yes, Mathfox is correct, if there is turbulence the pilot may opt to fly a little lower to give himself a better range within those speeds.

Given enough altitude, recovery from stalls is always possible--lower the nose to gain flying speed. I have had stall training from day one of flight school to my simulator check ride last year, so all airline pilots are familiar with stall recovery. Generally a pilot may only lose a few hundred feet in recovering from an approach to a stall (in the 737 the yoke will vibrate to warn of an impending stall). It may take a 10,000 foot loss (in a very extreme situation) to recover a jet from a stall, but at 35,000 feet there was plenty of altitude. If they got into an updraft, they would point the nose down to lose the altitude they might have gained if they thought they might exceed the maximum altitude of the aircraft. As I stated before, no pilot willingly flies through a thunderstorm. And yes, one of the reasons is the tremendous up and down drafts that occur inside some of these cells. The aircraft radars are very good and it is fairly easy to tell where the cells are and fly between them or around them. Sometimes you can fly over them but some storms build to over 50,000 feet and no airliner is certified to go that high.

So I am not going to speculate on what might have happened, but undoubtedly there were a series of failures that led up to the catastrophe. Therefore you can remain secure that there is an extremely low probability of all those failures happening
again. And again as I stated before, every single pilot who might be confronted with a similar situation is running every possible scenario through his mind and asking himself what he might do under these or those circumstances.

I hope some of that helps and I answered your questions.

Hutch :tiphat:

[stephiewd]

I'm confused a little now about the safety of storms. A few years ago - flying over the Bay of Bengal the jet I was on experienced turbulance that was bad enough that the airline staff had to take to their seats. I saw lightening flashing out of the window - so it must have been a storm we went through or close to.

I live under the flight path of Basel airport in Switzerland - we get some summer storms here too. A few weeks ago one whipped up from nowhere. About 5 mins before it really broke - as the winds were getting up and the thunder was rumbling - a plane came in to land.

Then the winds went wild and hailstones the size of golfballs pelted the area. It seemed to me the plane coming into land cut it pretty fine there.....

How do the pilots know how bad a storm is and where it is? Do they sometimes think let's go for it and get through?

Sorry - the media speculation has got to me - and just as the ice on wings got to me in February after Buffolo - now the prospect of my up and coming 2 summer flights especially if I hear thunder rumbling - is frightening me.

[Barb-SAN]

Okay you guys, I'll bite. You weren't supposed to be reading the media reports, remember? :shakehead (well, that's what we're here for!)
:lol::lol: I know, we are bad. :shakehead:blush:

And again as I stated before, every single pilot who might be confronted with a similar situation is running every possible scenario through his mind and asking himself what he might do under these or those circumstances.
We would probably have fewer automobile accidents if every auto driver spent as much time and energy analyzing car accidents as airplane pilots do for airplane accidents.:thumbsup:

I hope some of that helps and I answered your questions.
Yes, thank you very much. :)

[MathFox]

I'm confused a little now about the safety of storms. A few years ago - flying over the Bay of Bengal the jet I was on experienced turbulance that was bad enough that the airline staff had to take to their seats. I saw lightening flashing out of the window - so it must have been a storm we went through or close to.
The plane can easily handle turbulence that makes people that are not strapped in hit their head on the ceiling overhead bin. To prevent injuries to passengers and flight attendants it's best to have everyone strapped in when serious turbulence is suspected. (As I said in another post, have all lose items in the galley properly stowed.)

I live under the flight path of Basel airport in Switzerland - we get some summer storms here too. A few weeks ago one whipped up from nowhere. About 5 mins before it really broke - as the winds were getting up and the thunder was rumbling - a plane came in to land.

Then the winds went wild and hailstones the size of golfballs pelted the area. It seemed to me the plane coming into land cut it pretty fine there.....

How do the pilots know how bad a storm is and where it is? Do they sometimes think let's go for it and get through?
There is weather radar both in the plane and at the airport. Pilots and ATC know exactly where the shower is and the pilots of that particular plane knew they would be on the ground before the shower was over the airport. (The plane behind them would have waited till the shower was out of the way.) I have listened to pilots and ATC discussing the best way to fly around storms on several stormy nights in Amsterdam (via liveatc.net)

Sorry - the media speculation has got to me - and just as the ice on wings got to me in February after Buffolo - now the prospect of my up and coming 2 summer flights especially if I hear thunder rumbling - is frightening me.
You are not over your fears yet... Ice played no role in the Buffalo crash! Pilots got a reminder that thunderstorms are potentially dangerous (but they can fly around the bad parts, thanks to weather radar.) Pilots don't want their coffee cups jump in their laps, so they avoid turbulence... also for the comfort of the passengers.

[tusphotog]

How do the pilots know how bad a storm is and where it is? Do they sometimes think let's go for it and get through?


Simple. The onboard weather radars have a variety of pretty colors to depict precipitation (water). Here's a perfect example: http://www.airliners.net/photo/LTU---Lufttransport-Unternehmen/Airbus-A320-214/0176187/L/ (sorry, the image won't embed) The darker the color, the more water. More water means it's more powerful/severe of a storm.

I don't know if the radar onboard a plane is like ground based doppler radar (http://en.wikipedia.org/wiki/Doppler_radar)which maxes out in white. But aircraft radar has color shades which go from light green, dark green, yellow, and red. Areas that are red, are avoided at all costs. On the ground based radar returns, purple and white are usually associated with tornadic activity.

Having been in a "purple" cell, I can tell you that on the ground it's amazing to see that much water come pouring out of the sky. It's like standing under a fire hose.

[Kimmar]

Thanks for the quick reply, I've learned alot, (again!). I've been avoiding the media reports and for some reason thought this one was OK...Silly me! I should know better than to read anything about this right now because until they get the black box, or an eye witness it's *all* speculation! My bad...:angel:

I feel better again. I think my question should have waited until I read the official report and if *that* said "High altitude stall caused the failure" *then* I could have posted. I'll do better next time, since I'm only 2 years into my recovery, after a 25 year hiatus from flying...LOL

Thanks for the help, it's much appreciated!

[Barb-SAN]

Lots of discussion over on PPRuNe now that the black box has been found. This was posted today (a press release from Air France) http://corporate.airfrance.com/en/press/news/article/item/af-447-reaction-dair-france-a-la-note-dinformation-du-bea/ (bolding mine)
AF 447 - Air France’s reaction to the BEA’s information report
Friday 27 May 2011
On the eve of second anniversary of the AF447 tragedy, Air France and its staff are turning their thoughts to the families of the passengers and crew and wish to express their full solidarity.
The perserverance of the authorities, Airbus and Air France has led to the flight recorders and parts of the aircraft being found after a two-year search. The French Accident Investigation Bureau (BEA) is now able to reveal the sequence of events leading to the crash of flight AF447 from Rio to Paris on 1 June 2009.

This description of the facts therefore replaces the assumptions that have been made over the past two years.

It appears that the flight deck crew was monitoring the changing weather conditions and thus altered the flight path, that the initial problem was the failure of the speed probes which led to the disconnection of the autopilot and the loss of the associated piloting protection systems, and that the aircraft stalled at high altitude. It also appears that the flight captain quickly interrupted his rest period to regain the cockpit. The crew, made up of three skilled pilots, demonstrated a totally professional attitude and were committed to carrying out their task to the very end and Air France wishes to pay tribute to them.

All the data collected must now be analyzed. It will only be at the end of this complex task, which requires patience and precision, that the BEA will be able to establish the causes that led to the disaster.
We can already see that the authorities, the manufacturer and the airline have taken measures to avoid the repetition of such an accident.
Air France hopes that everyone has the patience to wait for the interim report that the BEA will publish in a few weeks, no doubt along with additional recommendations. The safety of the global air transport industry will be even stronger.

[Captain Hutch]

Hi Guys,
I'm back from a bit of hiatus--time in Mexico, working on articles, research....

I will give a few comments and then it would be best to wait for the analyzation of the black box data. The loss of air speed indications means the pilots have to rely on engine settings to determine airspeed. The key is recognition--if one or both pilots recognizes that the airspeed indication is erroneous, he has to be able to set a reasonable power to keep the aircraft flying in the particular realm that it has been in. Unable to do this for whatever reason, it's possible that a high altitude stall would follow. Since at this point I don't know which indicators failed and if there were any standby systems available, I will stop here. It does appear from the press release that fixes have already been applied and doubtless a few more will be put into place when the full analysis has been completed.

Hutch :tiphat:

[Barb-SAN]

Hi Guys,
I'm back from a bit of hiatus--time in Mexico, working on articles, research.... Hutch :tiphat:
Sounds like an interesting trip. We're glad to have you back...:)