Is Pilot Error to Blame for Icing Condition Accidents?

[mgb_phys]

The thing with ice build up is that you should definitely see a noticable change in the handling qualities of the aircraft. It will get more and more sluggish/unresponsive. If this starts to happen, you know you have an icing problem.

The BBC is reporting that the plane was on autopilot, so just like the ATR72 a few years ago. The autopilot is struggling with the ice and when the plane gets unflyable it just drops out, hands the plane back to the pilot and says - your problem!

From other blogs it appears that switching to manual in icing is a good idea but not required.

 

[Ivan Seeking]

It seems that the deicing system was on the entire time.

The National Transportation Safety Board investigator Steve Chealander says deicing was turned on 11 minutes into the flight after it left Newark headed to Buffalo.

http://www.publicbroadcasting.net/wbfo/news.newsmain?action=article&ARTICLE_ID=1470165&sectionID=1

 

[Cyrus]

Just because a system is on -and working, does not mean it will work. If the rate of ice build up is too large for the de-ice system to cope with, ice is still going to build up on the aircraft.

Major pilot error, IMO.

 

[Ivan Seeking]

Just because a system is on -and working, does not mean it will work. If the rate of ice build up is too large for the de-ice system to cope with, ice is still going to build up on the aircraft.

What is a pilot supposed to do if the rate of ice accumulation exceeds the rate of deicing?

 

[Cyrus]

What is a pilot supposed to do if the rate of ice accumulation exceeds the rate of deicing?

Don't fly into ice. Ice is very bad. Ice kills. The Maryland State Trooper rescue helicopter isn't ice equiped, so they can't fly in icy weather.

If you experience ice you change your altitude to see if it stops. If it doesn't stop you turn around and leave.

 

[Ivan Seeking]

Don't fly into ice. Ice is very bad. Ice kills. The Maryland State Trooper rescue helicopter isn't ice equiped, so they can't fly in icy weather.

Assuming that they were under the direction of air controllers, it sounds like there was nothing they could do. How were they going to know they were flying into severe ice? It almost sounds like the flight controllers were at fault. Or perhaps they couldn't tell that the conditions were as bad as they were?

 

[Cyrus]

Assuming that they were under the direction of air controllers, it sounds like there was nothing they could do. How were they going to know they were flying into severe ice? It almost sounds like the flight controllers were at fault. Or perhaps they couldn't tell that the conditions were as bad as they were?

No, you tell the controller you cannot comply with their instructions and have to divert. They would know because they shouldn't have left the autopilot on. Like I said, the controls would get very sluggish, and engine power would start creeping down. You can feel the ice on the airplane. The same way you can feel the pile of ice on your car if you don't clean it before you drive off. The pilot in command (PIC) is responsible for the safety of the aircraft, not the controller.

 

[Ivan Seeking]

No, you tell the controller you cannot comply with their instructions and have to divert. They would know because they shouldn't have left the autopilot on. Like I said, the controls would get very sluggish, and engine power would start creeping down. You can feel the ice on the airplane. The same way you can feel the pile of ice on your car if you don't clean it before you drive off. The pilot in command (PIC) is responsible for the safety of the aircraft, not the controller.

How long does it take for ice to accumulate to dangerous levels in the worst conditions?

 

[Cyrus]

How long does it take for ice to accumulate to dangerous levels in the worst conditions?

I've never flown in ice (nor do I want to try it), but from what I recall reading, it can take a few minutes. Then again, I'm not flying in bad weather all the time like the pros, so I'm sure they are trained to deal with worse weather.

According to my Jeppsen book:

In extreme cases, it can take as little as 5 minutes for 2 to 3 inches of ice to accumulate on the leading edge of the airfoil... Some aircraft may experience as much as 50 percent decrease in lift after the build up of only 1/2" of ice.

The problem with ice is that it changes the shape of the airfoil at the leading edge. Most of the lift is created at the first 1/3 of the airfoil section, so its very important that this section is clean and as designed.

 

[Ivan Seeking]

So while in auto-pilot, it would be fairly easy to get a significant accumulation in a minute or two, without knowing it.

I did hear that at the last moment, they tried to change to a new vector.

 

[Astronuc]

Pilot's actions scrutinized in Flight 3407 crash
http://news.yahoo.com/s/ap/20090216/ap_on_re_us/plane_into_home

Chealander said information from the plane's flight data recorder indicated that the aircraft pitched up at an angle of 31 degrees in its final seconds, then pitched down at 45 degrees.

The plane rolled to the left at 46 degrees, then snapped back to the right at 105 degrees — 15 degrees beyond vertical.

Radar data shows Flight 3407 fell from 1,800 feet above sea level to 1,000 feet in five seconds, he said. Passengers and crew would have experienced G-forces up to twice as strong as on the ground.

The plane crashed belly-first on top of a house about six miles short of Buffalo Niagara International Airport, two to three minutes from when it should have touched down on the runway.

Just before they went down in a suburban neighborhood, the pilots discussed "significant" ice buildup on their wings and windshield. Other aircraft in the area told air traffic controllers they also experienced icing around the same time.

Chealander said in an interview that the pilot may have rejected federal safety recommendations and the airline's own policy for flying in icy conditions by leaving the autopilot on even after he notified air traffic control that the flight crew had spotted ice on the leading edge of the wings and the windshield.

The Dash 8 Q400 plane, operated by Colgan Air, was equipped with a "stick shaker" and "stick pusher" mechanism that rattles the yoke to warn the pilot if the plane is about to lose aerodynamic lift, a condition called a stall. If not corrected in time, the mechanism automatically pushes the stick forward to avert a stall.

Chealander said the plane was on autopilot until the "stick shaker" and "stick pusher" kicked in, automatically putting the plane back in the pilot's hands.

At some point, the pilot switched on an anti-stall device that increases the speed of the plane by 20 knots and gives a pilot more margin to recover from a stall if it occurs.

Asked whether the pilot might have overreacted by pulling the stick back when it automatically went forward, Chealander said, "Yes, it's possible."

Still, he was careful not to be critical of the pilot.

"Everything that should have been done was done, so we keep looking," he said. "We keep looking, trying to find out why this happened."

Chealander said the plane's deicing system was turned on 11 minutes after it took off from Newark, N.J., and stayed on for the entire flight. Indicator lights showed the system appeared to be working.

He said the pilot was being "very conservative" by turning it on so soon.
. . . .

One key question would seem to be why the plane pitched so violently? The condition of the plane seem to change abruptly.

Also, did he leave auto-pilot on too long? He did activate the anti-stall system.

 

[FredGarvin]

It's not looking good for the pic for this crash. It's looking more and more like he may have overcorrected when he realized what may be happening. While no "illegal" to have the autopilot on in icing conditions, I liken it to having your cruise control on in rush hour. Not a bright move.

 

[Astronuc]

The latest I heard is a concern that the pilot let the air speed decrease too much, and at the point when he should have put the nose down to get air speed, he increased power but pulled back on the stick, and the plane apparently went into an unrecoverable stall - and subsequently pancaked into the house.

Could the icing have affected the air speed indicator?

I wonder why the air speed would have been too low?

 

[turbo]

I wonder why the air speed would have been too low?

Too slow for conditions probably. If the wings were deformed from icing, the pilot would probably have to increase speed to generate enough lift to stay in the air. I wouldn't want to be the one at the controls trying to guess what the plane's stall speed is with iced wings.

 

[mgb_phys]

Could the icing have affected the air speed indicator?

Icing is a problem so the pitot tubes have electric heaters

 

[Ronnin]

I heard a pilot saying that too much flap in this situation could lead to a tailstall. I thought extended flaps anytime you need more lift.

 

[edward]

The plane was on auto pilot until just before the crash. Auto pilot has a feature commonly called "stick shaker stick pusher"

As a plane approaches stall speed the Yoke will vibrate then within seconds if no action is take by the pilot the yoke will push forward to increase speed.

There is now some speculation that the pilot may have been alarmed by the forward motion of the yoke and pulled back too far resultiing in the initial unexplained 30 degress + upward nose pitch.

 

[Danger]

It seems to me that everyone is overlooking something. You're all concentrating on icing affecting the wings and lift. Let's not forget that it takes even less ice to screw up the horizontal stabilizer, which controls attitude. It's possible that the pilot had no pitch control and didn't realize it until George kicked out and handed it back to him.
If the ice had also jammed the elevators, he might have horsed back on the yoke and broken it loose, which might have resulted in a higher nose position than intended.

 

[russ_watters]

I wonder why the air speed would have been too low?

Did the airspeed decrease before or after he pulled back on the stick? With the aerodynamics destroyed, there is a lot of room to speculate on what the plane would do, but the ice could have contributed to the violence of the pullup and if the pullup caused an immediate stall (a stall is caused by a too-high angle of attack, not a low speed), the airspeed would drop rapidly and the plane would go from flying to falling like a brick in just a few seconds. I'm interested to see how much detail we get about what the plane was doing those few seconds - it may be quite revealing.

And once the airspeed gets pretty low, there is no possibility for control left. The violent rolling and pitching might not be what you would envision a flying airplane to do - it could be more like a leaf or a piece of paper fluttering to the ground.

 

[mgb_phys]

With the aerodynamics destroyed, there is a lot of room to speculate on what the plane would do,

The stick-shake stall warnign is presumably based on the design stall speed of the plane, not the stall speed of the new ice+aerofoil wing.
Is there any way for the system sense how close you are to stall from the real time response of the plane to controls?

 

[edward]

The stick-shake stall warnign is presumably based on the design stall speed of the plane, not the stall speed of the new ice+aerofoil wing.
Is there any way for the system sense how close you are to stall from the real time response of the plane to controls?

I don't think so that is why they recommend that the pilot be in control during icing conditions. The auto pilot compensates for conditions a pilot would have already felt in the controls.

There is also an issue with the landing gear being lowered was it the pilot or the autopilot?

Apparently it was the pilot who raised the gear again a few seconds later.

 

[Cyrus]

WOW!

45 degree turn WHILE a 45 degree nose down IS INSANE!

When we practice 45 degree turns we call it 'steep turns' and it is STEEEP. YOU FEEL THE G's in that kind of a turn. They were CRANKING that airplane LIKE HELL. They could have very easily just ripped the god damn wings off!

Danger: Thats a GREAT analysis I didn't think of! I emailed it to my professor at NASA langely. I'll post his reply when he sends it. He does crash analysis as well on the side. I'll also ask my helicopter professor who also does crash stuff.

 

[WhoWee]

What is the difference between a "wing stall" and a "tail stall" and how should you correct each...I heard an aviation expert discussing it on the radio and I didn't catch all of the details...I thought he said the correction of one is the opposite of the other?

 

[FredGarvin]

http://aircrafticing.grc.nasa.gov/courses/inflight_icing/related/3_2_3f_RI.html

The gist with a tail stall is related to flap usage because the wings are still producing lift with the decreased speed but the tail can not. It seems like a design flaw to have a portion of the flight envelope that can have the wings still producing lift and the tail section not. The icing in this case could have created that in that the icing screws up the aero of the tail so it stalls sooner than it normally would. The cool things about the video above are:
1) The statement that even though you correct very quickly, the aerodynamics has a lag and takes some time. Hence the statement that if you have this happen on approach, you're toast.
2) The flight video shows a stalled stabilizer in flight.
3) You can see how violent the yoke can become in a real stall.

MGB...to answer your question, NO they are calibrated for a clean wing.

Here's a nice quote that shows, I think, that the pilot had the right idea, but the wrong implementation:

William Rieke, a NASA research pilot and chief of aircraft operations at NASA Glenn, says to consider the 180-degree turn only if you have no other options -- like an airport in front of you. "If you are getting ice and you have to do a 180 to get out of it and in the process go back through [the ice] and accumulate more -- or worst case, if the air mass is moving in the direction you just turned to, you may be in it even longer than your initial encounter." If terrain clearance or clouds were not an issue, Rieke would prefer to descend 3,000 feet or, if the airplane was powerful enough to climb with the weight and drag of accumulated ice, climb 3,000 feet in an effort to get out of the icing conditions. (Most training aircraft would not be able to climb if they were carrying an appreciable quantity of ice.)

They were at a low altitude within 7 miles of the airport. Undoubtedly the guy was lining up for final. He was thinking just get this thing down ASAP. Unfortuantely ASAP in this case was too soon. This will be supported when they release the configuration of the airplane and it's speed just prior to the crash.

http://flighttraining.aopa.org/ft_magazine/fullstory.cfm?id=5222&issue_title=February

 

[edward]

Radical steep turns are sometimes used to dislodge the ice from wings. Even the B-1 bomber pilots were taught to use the method. They usually crashed. The B-1, built to fly at supersonic speed and at tree top level over Siberia had no deicing system.

 

[rcgldr]

http://aircrafticing.grc.nasa.gov/courses/inflight_icing/related/3_2_3f_RI.html

The gist with a tail stall is related to flap usage because the wings are still producing lift with the decreased speed but the tail can not.

Listen to the chat in the actual test part of the video. The recovery procedure was to retract the flaps which resulted in near instant recovery. The issue wasn't speed (not with a pitch down attitude more than -10 degrees), but that the deployed flaps were interfering with the tail control at that air speed for that particular aircraft. This could be due to the air flow at the tail being interfered with, or because increasing camber by deploying flaps increases the pitch down torque on the wings.

update - The issue was speed related in the sense that the elevator could not provide enough sufficient negative lift (upwards force) to prevent the aircraft from pitching downwards, even at maximum non-stall deflection. The situation is made worse by the pilot increasing the elevators angle of attack so that it stalls (due to angle of attack, not air speed). In the video, the flaps are deployed while the aircraft has a pitch of -13 degrees. The pilot continously pulls back on the elevator to reduce the amount of negative pitch, which decreases the air speed. The flaps produced enough drag that even with pitch around -9 degrees, the airspeed continues to decrease as the pilot feeds in more up elevator until the elevators angle of attack becomes excessive and stalls (which is independent of air speed), severely reducing the negative lift at the tail and allowing the aircraft to quickly pitch downwards to -30 degrees. Retracting the flaps reduces the camber related pitch down torque on the wings, and allows the aircraft to pick up speed more quickly, restoring control authority to the elevator. The downwards diversion of air from the wings with fully extended flaps could also be reducing the air speed at the tail. This is why some aircraft use high "T" tails to keep the control surfaces out of the downwash from the wing.

For a normal aircraft, the center of gravity is front of the center of pressure, and the tail generates less lift or negative lift in order to maintain pitch angle. It's a form of self-stability in that a given elevator angle will correspond to a given air speed. For a given elevator angle, too much speed pitches aircraft up, slowing it down, too little speed pitches it down, speeding it up, and the aircraft will tend to fly at a particular air speed for a given elevator angle (assuming power output is not changed).

A tail stall means the tail can't generate sufficient negative lift, so the air craft pitches down as shown in the video above. This wouldn't explain the roll reaction described in the actual accident.

Although icing could have been an issue, I've wondered if the accident could have been due to faulty flap and/or air brake deployment, either malfuction (one flap deploying more than the other, or both deploying too much plus air brakes), or pilot error. Another possibility is that the de-icing system failed, and if the pilot tried to reduce air speed to normal flight mode, the aircraft would have stalled (wing stall).

I got the impression that things didn't go bad until the flaps were deployed. As far as auto-pilot usage goes, in zero visibility conditions, the military will use auto-pilot to do the approach and landings (called zero-zero), and some commercial airliners have the same auto-plilot landing capability.

http://en.wikipedia.org/wiki/Autopilot

 

[FredGarvin]

Listen to the chat in the actual test part of the video. The recovery procedure was to retract the flaps which resulted in near instant recovery. The issue wasn't speed (not with a pitch down attitude more than -10 degrees), but that the deployed flaps were interfering with the tail control at that air speed for that particular aircraft. This could be due to the air flow at the tail being interfered with, or because increasing camber by deploying flaps increases the pitch down torque on the wings.

Umm...the issue is somewhat about speed. All aircraft with flaps have Vne's with the flaps deployed in each position which is usually quite a bit slower than most cruising speeds. The fact that the flaps are deployed means that you are operating at reduced speeds.

Did you listen to the interview at the beginning? It was said that the corrective action was applied in about .2 seconds and it too "a few seconds" to recover. I counted about 3 seconds between pitch down and recovery and that's with someone with their hands on the flap controls. That is not instantaneous. They lost 300 feet in altitude and they knew exactly when they were going to induce the stall. Now put someone in the situation where they don't know it's coming and add a much delayed reaction time plus, probably, a lot worse condition of icing.

 

[Cyrus]

http://video.google.com/videoplay?docid=2238323060735779946&ei=HCSeSZaFIKGG_AG94qjsBg&q=nasa+icing&hl=en

Gooooooooooooooooooood video.

 

[Cyrus]

Umm...the issue is somewhat about speed. All aircraft with flaps have Vne's with the flaps deployed in each position which is usually quite a bit slower than most cruising speeds. The fact that the flaps are deployed means that you are operating at reduced speeds.

Did you listen to the interview at the beginning? It was said that the corrective action was applied in about .2 seconds and it too "a few seconds" to recover. I counted about 3 seconds between pitch down and recovery and that's with someone with their hands on the flap controls. That is not instantaneous. They lost 300 feet in altitude and they knew exactly when they were going to induce the stall. Now put someone in the situation where they don't know it's coming and add a much delayed reaction time plus, probably, a lot worse condition of icing.

Based on the NASA video, Jeff is exactly right.

 

[Danger]

What is the difference between a "wing stall" and a "tail stall" and how should you correct each... ...I thought he said the correction of one is the opposite of the other?

The one main point of confusion to groundhogs is that the tailplane produces a negative lift to counteract the aeroplane's natural tendency to pitch down. Most people assume that it acts in the same lift fashion as a main wing. In a wing stall, you want to drop the nose and/or add power in order to recover because your angle of attack is too high. In a tail stall, you have to pull up to regain proper attitude because the angle is too low.

 

[rcgldr]

Listen to the chat in the actual test part of the video. The recovery procedure was to retract the flaps which resulted in near instant recovery. The issue wasn't speed (not with a pitch down attitude more than -10 degrees), but that the deployed flaps were interfering with the tail control at that air speed for that particular aircraft. This could be due to the air flow at the tail being interfered with, or because increasing camber by deploying flaps increases the pitch down torque on the wings.

update - The issue was speed related in the sense that the elevator could not provide enough sufficient negative lift (upwards force) to prevent the aircraft from pitching downwards, even at maximum non-stall deflection. The situation is made worse by the pilot increasing the elevators angle of attack so that it stalls (due to angle of attack, not air speed). In the video, the flaps are deployed while the aircraft has a pitch of -13 degrees. The pilot continously pulls back on the elevator to reduce the amount of negative pitch, which decreases the air speed. The flaps produced enough drag that even with pitch around -9 degrees, the airspeed continues to decrease as the pilot feeds in more up elevator until the elevators angle of attack becomes excessive and stalls (which is independent of air speed), severely reducing the negative lift at the tail and allowing the aircraft to quickly pitch downwards to -30 degrees. Retracting the flaps reduces the camber related pitch down torque on the wings, and allows the aircraft to pick up speed more quickly, restoring control authority to the elevator. The downwards diversion of air from the wings with fully extended flaps could also be reducing the air speed at the tail. This is why some aircraft use high "T" tails to keep the control surfaces out of the downwash from the wing.

The main point of my previous post is that a stall is the result of excessive angle of attack, not airspeed.

http://en.wikipedia.org/wiki/Stall_(flight)

In a wing stall, you want to drop the nose and/or add power in order to recover because your angle of attack is too high. In a tail stall, you have to pull up to regain proper attitude because the angle is too low.

Update - you're correct, I thought "tail stall" mean a true stall of the elevator, but it doesn't. See my post below.

I've corrected this post to read "true stall of elevator".

In both cases, the angle of attack is too high. "True stall of elevator" only occurs when the elevator is excessively deflected upwards (or downwards if inverted flight, I'm ignoring inverted flight for the rest of this) quite a bit. Pulling up further will just worsen the situation. The recovery procedure is to feed in down elevator and/or increase air speed, reducing the angle of attack.

An extreme example of wing stall is a snap roll induced through excessive up elevator at speed without any aileron input. I flown some radio control models that do this, you peg the elevator stick back and instead of climbing, the model just rolls, a true snap roll (one wing stalls a bit before the other, resulting in a very fast roll). A not so obvious case occurs when ground towing a glider. The glider will snap roll if excessive up elevator is used. The first response to any roll reaction during a ground tow, both real and model gliders, is down elevator and then aileron to correct since there isn't enough time to figure out if the issue is a normal roll or snap roll. It's more of an issue for model gliders since there are contests (F3J) where minimum time during launch is a goal, so very high towing forces are used (over 20 g's). Snap rolls can also unintentionlly result from pylon racing models, when the high g turns cause the main wing to stall.

 

[Danger]

Jeff, I have the utmost respect for you as a scientist and educator, but I'm talking about real aircraft as opposed to toys.
A tail stall refers to the inability of the horizontal stabilizer to provide proper trim. That results in a nose-down pitch attitude. Pushing forward on the stick, as you would for a wing stall, will send you straight into the ground.

 

[Cyrus]

update - The issue was speed related in the sense that the elevator could not provide enough sufficient negative lift (upwards force) to prevent the aircraft from pitching downwards, even at maximum non-stall deflection. The situation is made worse by the pilot increasing the elevators angle of attack so that it stalls (due to angle of attack, not air speed). In the video, the flaps are deployed while the aircraft has a pitch of -13 degrees. The pilot continously pulls back on the elevator to reduce the amount of negative pitch, which decreases the air speed. The flaps produced enough drag that even with pitch around -9 degrees, the airspeed continues to decrease as the pilot feeds in more up elevator until the elevators angle of attack becomes excessive and stalls (which is independent of air speed), severely reducing the negative lift at the tail and allowing the aircraft to quickly pitch downwards to -30 degrees. Retracting the flaps reduces the camber related pitch down torque on the wings, and allows the aircraft to pick up speed more quickly, restoring control authority to the elevator. The downwards diversion of air from the wings with fully extended flaps could also be reducing the air speed at the tail. This is why some aircraft use high "T" tails to keep the control surfaces out of the downwash from the wing.

The main point of my previous post is that a stall is the result of excessive angle of attack, not airspeed.

http://en.wikipedia.org/wiki/Stall_(flight)

In both cases, the angle of attack is too high. Tail stalls only occur when the elevator is excessively deflected upwards (or downwards if inverted flight, I'm ignoring inverted flight for the rest of this) quite a bit. Pulling up further will just worsen the situation. The recovery procedure is to feed in down elevator and/or increase air speed, reducing the angle of attack.

An extreme example of wing stall is a snap roll induced through excessive up elevator at speed without any aileron input. I flown some radio control models that do this, you peg the elevator stick back and instead of climbing, the model just rolls, a true snap roll (one wing stalls a bit before the other, resulting in a very fast roll). A not so obvious case occurs when ground towing a glider. The glider will snap roll if excessive up elevator is used. The first response to any roll reaction during a ground tow, both real and model gliders, is down elevator and then aileron to correct since there isn't enough time to figure out if the issue is a normal roll or snap roll. It's more of an issue for model gliders since there are contests (F3J) where minimum time during launch is a goal, so very high towing forces are used (over 20 g's). Snap rolls can also unintentionlly result from pylon racing models, when the high g turns cause the main wing to stall.

No, you are wrong. Danger is right. Watch the NASA video I linked.

 

[rcgldr]

A tail stall refers to the inability of the horizontal stabilizer to provide proper trim. That results in a nose-down pitch attitude.

I wasn't aware that the term "tail stall", isn't the same as a true elevator stall. After doing a web search, a "tail stall" refers to the case where external forces deflect the elevator downwards, yanking the yoke forwards, generally coinciding with deployment of flaps at high speed (probably related to the pitching torque I mentioned before). The pilot has to pull back on the yoke with a lot of force, to overcome the external forces on the tail, to recover the elevator back to a normal position. Although this is not a true "stall" situation (excessive angle of attack on the elevator) it's called a "tail stall".

In the NASA video, it appears that a true elevator stall is induced. Full flaps are deployed at high speed and the pilot states that he applies up to 80 pounds of pulling force on the yoke (he's pulling on a force sensor connected to the yoke) to oppose the external downwards force on the elevator, allowing the pilot to continue to control the aircraft while in a "tail stall" situation. In this case, the pilot continues deflecting the elevator further upwards as the aircraft slows, apparently acheiving a true stall at the elevator, resulting in the aircraft suddenly pitching down to -30 degrees while the yoke is held in place by the pilot (as opposed to being yanked forward). Note that the pilot retracted the flaps as soon as the true stall was detected in order to recover.

I don't know if icing was an issue in the NASA video, but ice accumulation on the horizontal stabilizer and elevator can result in a true stall of the elevator occurring at a lower angle of deflection than normally required to cause a stall on a "clean" tail section.

Full flap deployment at speed is an issue in the design of aircraft. During flight testing of a certain GA aircraft intended for certification, the first time that Norm Howell dropped the flaps, the horizontal tail stalled. (I don't know if this was a true stall or just insufficient negative lift due to inadequate elevator). Norm says that the airplane instantly went over on its back and started an inverted spin. Cool Norm reached over, retracted the flaps and then recovered from the spin. Eventually, in the wind tunnel we found that the airplane needed a cambered (upside down) airfoil on the horizontal tail.:

http://yarchive.net/air/tail_stall.html

Recovering from a tail stall: Here is how to recover from a tail stall: Immediately raise flaps to the previous setting. Pull aft on the yoke. Copilot assistance may be required. Reduce power if altitude permits; otherwise maintain power. Do not increase airspeed unless it is necessary to avoid a wing stall.:

http://www.aopa.org/asf/publications/sa11.pdf

 

[Danger]

I wasn't aware that the term "tail stall", isn't the same as a true elevator stall.

That would certainly explain the original disagreement. My apologies if I seemed condescending.