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!Cyrus said: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.
http://www.publicbroadcasting.net/wbfo/news.newsmain?action=article&ARTICLE_ID=1470165§ionID=1The National Transportation Safety Board investigator Steve Chealander says deicing was turned on 11 minutes into the flight after it left Newark headed to Buffalo.
Cyrus said: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.
Ivan Seeking said:What is a pilot supposed to do if the rate of ice accumulation exceeds the rate of deicing?
Cyrus said: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.
Ivan Seeking said: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 said: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 said:How long does it take for ice to accumulate to dangerous levels in the worst conditions?
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.
One key question would seem to be why the plane pitched so violently? The condition of the plane seem to change abruptly.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.
. . . .
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.Astronuc said:I wonder why the air speed would have been too low?
Icing is a problem so the pitot tubes have electric heatersAstronuc said:Could the icing have affected the air speed indicator?
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.Astronuc said:I wonder why the air speed would have been too low?
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.With the aerodynamics destroyed, there is a lot of room to speculate on what the plane would do,
mgb_phys said: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?
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.)
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.FredGarvin said: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.
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.Jeff Reid said: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.
FredGarvin said: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.
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.WhoWee said: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?
Jeff Reid said: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 - you're correct, I thought "tail stall" mean a true stall of the elevator, but it doesn't. See my post below.Danger said: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.
Jeff Reid said: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.
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".Danger said:A tail stall refers to the inability of the horizontal stabilizer to provide proper trim. That results in a nose-down pitch attitude.
That would certainly explain the original disagreement. My apologies if I seemed condescending.Jeff Reid said:I wasn't aware that the term "tail stall", isn't the same as a true elevator stall.