Operational or Real World Questions about Icing

[2550]

Yes, intercepting from above isnt a good idea for a few reasons. The long straight in seems like about the best bet for me for this hypothetical really bad day. Im curious to hear your plan. That is, as long as its not one that would require super human skills to pull off. After all, if this happened your already not doing to well.

The other thing to remember is that its not likely a totally uniform rate of catch all the way from 10000 down to the gound. Should be better in some places and worse in others.

I suppose assuming a radar environment you could ask for vecors for a steep decent onto a shorter ILS. I always forget that in most places you have the option of getting some direction from the ground. You d also want to get any pireps you could for better or worse altitudes, and to see if there would be less ice arriving from the other side of the airport.

[photofly]

After a bit of online research and calculation, the stagnation temp at Mach 0.3 is about 2% higher than the free stream temperature. So a rise of about 4-5 degrees.

If you think there's frictional heating in the boundary layer then you'll estimate a higher temperature rise. But the leading edge of the wing, where the high pressure is, is near the stagnation line so I don't think there's much viscous heating happening there.

On the top surface of the wing there's a low pressure area so I'd expect a lot of cooling instead.

It's true you don't have any control of the mechanism but if you have an idea of what causes it you have a chance of estimating it reasonably well.

[photofly]

It kind of supports that frictional heating is a significant factor.

i guess it depends where on the wing or airframe you measure the temperature.

The set of slides you posted earlier was very interesting. That was talking about aerodynamic heating relative to the stagnation temperature; later in the slides the author pointed out that his analysis was only valid away from the leading edge.

[Colonel Sanders]

a steep descent onto a shorter ILS

That would be my choice. It's by no means the only
one, but the objective is to minimize time spent in
the icing. Stay above the layer on the localizer until
close in, then descend wings level at a very high rate
of descent - even 5000 fpm might not be enough - so
that you catch the correct lobe of the glideslope. I
would not lower the gear. 200 mph after GS with
the needles centered down to 200 AGL, and no
go-around. Throttle idle, gear and flap after visual
at 200 AGL. Should be a long runway, you will use
most of it.

5000 fpm sounds like a lot, but it isn't really. This
is not a normal maneuver. I suspect in excess of
10,000 fpm would be better but difficult for most
people to control. Altimeter lag would be annoying.
GPS altitude might be more precise.

[dr.aero]

photofly...

From ACWM - http://i.imgur.com/xH2i9Tf.jpg

Colonel...

A poster suggested intercepting the glideslope from above and you replied, "Careful about that", as if to imply that that wasn't what you were looking for. Then your suggestion is to do exactly that and suggesting that descending at 5000'/min might not be enough. Hmm...

What is the "correct lobe of the glideslope"?

[Colonel Sanders]

The one that gives you a 3 degree descent.

And that's why you have to be careful - what
you need to do, is descend to a specific (eg PT)
altitude beyond a specific distance from the
runway threshold - and only then intercept
the correct GS lobe, from underneath.

Like a turnaround after takeoff, I never said
this was going to be easy In fact, very
few pilots would be capable of an optimal
performance.

[dr.aero]

Colonel...

OK, I see what you mean.

It's not just aircraft that aren't properly equipped for icing that should do that - I do similar things when flying a turboprop. It makes no sense to descend below the 100 safe ALT to the PT ALT while you fly the procedure turn if you know you're going to be picking up a lot of ice. Depending on the approach, you can sometimes stay above the cloud at your 100 safe ALT and descend to your FAF ALT or MDA when you're flying PT inbound. Each scenario is different but if a full icing equipped airplane is doing that it puts it into perspective how bad icing can be!

[Colonel Sanders]

Shouldn't omit this:

http://en.wikipedia.org/wiki/American_Eagle_Flight_4184

[dr.aero]

Nor this:

http://www.youtube.com/watch?v=o5MK2HVdGxA

[old_man]

Even if your aircraft is certified known icing you can still get yourself in trouble.

I remember seeing this guy on a poster at an airport once. I don't know the whole story but I think they were close that day.

[Colonel Sanders]

I would suspect another encounter with "moderate" icing.

Note the buildup behind the boots. Incredible drag. As
I said before, if you're going to mess with icing, you'd
better have immense excess thrust. A marginally-powered
aircraft is simply going to do the "plummeting ice cube trick".

A lot of pilots I know are skeptical of boots. They might
be legal, but some bleed air on the leading edges - and some
higher speed for leading edge temp rise - works wonders.
Generally an aircraft like that has the performance to easily
get on top of the weather, and the icing in it.

What I haven't mentioned is that there are all sorts of weird
problems, when you pick up a load of ice.

I've mentioned drag - everyone thinks it's weight, but - you
need lots of power to handle the increased drag. That means
a normally aspirated piston engine won't do it. Doesn't have
the climb rate at higher altitudes to quickly get on top.

People are terrified when they get a 1/4 inch of ice on the
windshield, but I really don't care. Won't be the first time
I've landed without being able to see forward

You mess with ice, you might learn about the following:

1) with a piston engine, your crankcase breather tube
freezes up and plugs, and blows your front crank seal
which conveniently empties all your engine oil on your
windshield. Oh well, I guess it's hot, it might help melt
the ice on it.

2) fuel vents. I live in fear of my fuel vents plugging up.
You probably don't care, but if air can't get into your
tanks, after a while, fuel can't get out, even if the tank
collapses. My one mayday in my entire life was due to
fuel venting. Or not. PS Was mud daubers, not ice.

3) tailplane stalling. Lots of people load up with ice,
then they apply flaps, fly too slow and stall the tail,
which is really, really bad. You pick up a load of ice,
you want a long runway, absolute minimum flaps,
and carry a little extra speed because you really
don't know what your stall speed is any more.

Anyways, you people are probably sick to death
of hearing from me, but if one of the lurkers learns
something from reading this, it's worthwhile.

[Masters Off]

I'll count as one,

Thanks Colonel.

[PilotDAR]

I am certainly no expert on icing - I have only been descending out of control due to icing three times in my flying career. Once as a teenage passenger in a winter night flight in a 150, next as copilot flying a 3000 pound overgross (ferry fuel), non deiced Twin Otter, during a ferry flight - but that was over the Mediterranean in August, so I knew we'd melt it off somewhere on the way down. the worst an scariest was this one (from my memoirs on flying survival):

Bill and I were ferrying a Cessna 303 from Canada to England, for delivery to its new owner. With lots of Cessna 310 and 340 experience, I felt extremely comfortable flying the 303, and off we went. This particular aircraft was very well equipped, with full IFR equipment as one would expect, and full known icing equipment. So there we were, flying in IMC, though often with a view of the ground, but picking up ice. No problem, I just selected on all of the deicing equipment, and had a look around the aircraft to assure that is was functioning. The boots on the wings, and just the very tips of the horizontal stabilizer could be seen, and I was able to confirm that they were operating as expected. Obviously, the boot on the vertical fin could not be seen, and this was an act of faith.

After a while, and while obviously picking up some ice, a slight twitch in the yaw axis developed. It was about what you’d feel if you were alternatively pushing the pedals a little. I looked over at Bill’s knees, and asked, “Are you playing on the pedals down there?” But as I asked, I observed that his knees were still, so this was not his doing. Next I scanned the engine instruments – they indicated that the engines were both purring. The twitching in yaw got a little worse, and was now noticeable in pitch as well. Whatever it was that causing the twitching was making be nervous. When I’m nervous, I like to be closer to maneuvering speed (Va), in case something unexpected happens. So, I pulled the power back, and began to slow down…

As the plane slowed, we were suddenly rodeo riders, the plane was yawing and pitching violently, though roll control was prefect the whole time. Yaw was ten degrees either side uncontrollably, and pitch, though harder to estimate, was enough to give us quite a variation in “G”. Whatever the problem, slowing down made it a lot worse, so I sped up, and it settled down. The only thing it could be was airframe ice, nothing else would seem to have changed since we took off. But this was a known icing certified aircraft! So I flew as fast as I could, knowing that whatever it was, was getting worse, and we were still in the ice. At the higher speed, anything bad which happened, would happen worse, and faster! I had to get out of the ice.

We were able to descend, flying up the valleys in the mountains, not far from Wabush, Labrador. We were lucky enough to find warmer air, and the ice slowly shed on its own. An hour or so later, I landed in Shefferville, Quebec for fuel. Of course, slowing down, was an exercise in extreme caution. But the plane handled perfectly. The after landing visual inspection revealed no ice, or other defects at all. Mystery… Our trip continued….

Bill was flying the leg from Iceland to Scotland. I was bored. Searching for some new stimulus, I found the previously unread flight manual for the aircraft, and browsed. Among the commonly found white pages, was an uncommon fluorescent red one. To its corner, stapled a tiny zip lock bag, which contained a placard. My interest was peaked now (better late than never). The information on the page instructed that flight into known icing conditions was prohibited, and at the first encounter, an immediate 180 degree turn was to be executed. The placard in the zip bag simply said “Flight in icing conditions prohibited”. Well that was clear! But, with the placard in the bag, and the bag in the book, and the book in the glovebox, the pilot (who had not bothered to read the book prior to flying) had no way of knowing! To read on, it turns out that because the Cessna 303 has a “crucifix” tail, meaning the horizontal stabilizer is mid way up the vertical stabilizer, their respective leading edges form a cross. The middle of this cross was not deiced, and thus a block of ice would form there, and disrupt the smooth airflow over the tail. The result was (in several cases) fatal inflight breakup of the aircraft, due to loss of pitch and yaw control. This, I could imagine! This flight manual page, and placard were required by airworthiness directive 86-01-01.

The final instruction on the page was to install the placard. I did.

I understand that soon an electric pad was developed for installation on the offending leading edges, to correct this design deficiency.

I learned from that to read the flight manual before flying. I don’t know how close we came to breaking that plane up in flight, but it was a lot closer than we should have come!

[Colonel Sanders]

ice would form there, and disrupt the smooth airflow over the tail

You are lucky to be alive! Hopefully someone (maybe
more than one person) will take away from this discussion
the dangers of icing on the tail.

PS Ever read, "Fate is the Hunter" by Ernie Gann?

[dr.aero]

Colonel...

tailplane stalling. Lots of people load up with ice,
then they apply flaps, fly too slow and stall the tail,
which is really, really bad. You pick up a load of ice,
you want a long runway, absolute minimum flaps,
and carry a little extra speed because you really
don't know what your stall speed is any more.

It's actually the other way around - fly too fast and you'll stall the tail! A tailplane stall recovery requires aggressively pulling the nose back up and ensuring the speed doesn't increase. If the speed is allowed to increase as the plane nose-dives, the stall will get worse. On recovery you need to pull the nose up, reduce power (usually), and reduce flaps - all opposite (usually) to a wing stall recovery.

You do want minimum flaps and you also want to make sure you don't put the flaps out at their maximum speed - give a buffer of speed below the maximum speed for the flaps before you extend them.

Carrying a little extra speed is good because of ice formations on the wings that will increase stall speed, however, if you're concerned about tailplane stall as well you need to make sure that you don't go too fast. It's recommended to fly with the autopilot off so that you're able to "feel" the airplane. You might be able to feel the lack of control or oscillations of pitch as the tailplane approaches stall and be able to slow down before it stalls. But sometimes you don't get many symptoms before the nose aggressively pitches down.

[photofly]

fly too fast and you'll stall the tail!

Try and explain how that works, from your knowledge of stall as exceeding the critical angle of attack.

[gaamin]

NASA did some research on icing in general, and tailplane icing in particular. They made a good educational video :



JBL

[Colonel Sanders]

fly too fast and you'll stall the tail!

Really?! I guess that's never been an issue, because with
a load of ice, excessive airspeed is rarely a problem! Often
even with full power the aircraft may not maintain airspeed
and you have to descend.

[dr.aero]

photofly...

Try and explain how that works, from your knowledge of stall as exceeding the critical angle of attack.

It's a LOT easier with a whiteboard!

The tailplane is an inverted airfoil (compared to the wing) so it normally creates lift in the downward direction. That means that the faster air flows around the bottom of the tailplane so increasing angle of attack would be caused by increasing the downward deflection of air in front of the tailplane - flaps do exactly that. Extending flaps on a low wing airplane with a t-tail won't affect the tailplane angle of attack as much as a high wing Cessna.

Colonel...

Yup. In a lot of the piston airplanes it isn't as big of a problem if the airplane is covered equally around but in turbines that's not the case. You will still have tailplane stalling problems in any airplane if, for example, your boots are working on the wings but not on the tail.

This is from FAA AC 23.143-1. ICTS stands for Ice Contaminated Tailplane Stall.

When the leading edge of a horizontal tailplane becomes contaminated with ice accretions, control of the aircraft may be severely affected. The ICTS phenomenon in its extreme form may result in an uncontrollable nose-down pitching event. Pilots may sense ICTS or impending ICTS as one or a combination of:

- difficulty to trim in the pitch axis;
- a pulsing or buffeting of the longitudinal control; or
- a lightening of longitudinal control push force (or an increase in pull force) necessary to command a new pitch attitude.

ICTS typically occurs either while extending the wing trailing edge flaps to the landing position or with the flaps already extended to that position when operating in, or departing from, icing conditions. The flap extension increases the wing airflow downwash angle, reduces the aircraft angle-of-attack (AOA) for the same lift coefficient, and increases nose- down pitching moment which requires more tail download, all of which in turn result in an increased negative AOA at the horizontal tailplane.

This increased negative AOA coupled with an ice contamination on the horizontal tailplane leading edge causes turbulent or separated airflow, or both, on the undersurface of the horizontal tailplane, which can result in reduced or reversed elevator hinge moment, or a complete stall of the tailplane, or both. Since flaps are normally only extended to the landing position during final approach to landing, an ICTS event under these conditions has a high probability of being catastrophic due to the associated low ground clearance.

Note: Full flap extension is generally the only case that results in ICTS but this may
not be true for all designs, particularly those with closely spaced flap settings.

Tailplane stall has been reported on several airplane type designs with small quantities of ice observed on the airplane. In some cases, the critical ice accretion for susceptibility to ICTS has been a thin rough layer of ice on the protected and unprotected portions of the leading edge that accretes during the time that:

1. The airplane enters icing conditions.
2. The flightcrew recognizes the icing conditions and activates the ice protection system.
3. The ice protection system performs its intended function.

On airplanes with reversible control systems, the high trailing-edge-down elevator hinge moment translates to a forward motion of the longitudinal control and may result in recovery forces beyond the flightcrew's strength capabilities. ICTS reduces tail lift or diminishes the effectiveness of the primary and secondary control surfaces.

Diminished control effectiveness ranges from reduced elevator authority to sudden, large and unexpected changes in required control forces. Tab driven reversible pitch control designs may have insufficient authority to overcome the elevator hinge moments resulting from ICTS.

Tailplane stall can occur when:

- wing flaps are extended;
- engine power is increased;
- the pilot makes a nose-down control input;
- the airplane increases speed or encounters gust conditions; or
- a combination of these factors with flaps extended.

Pitch trim difficulty, forward pitch control force lightening or increased pull force to maintain pitch attitude, and pulsing or buffeting of the longitudinal control may not be recognized as cues of ICTS. The pilot may incorrectly interpret these cues as aerodynamic warning of an impending wing stall and perform the wrong, possibly catastrophic, corrective action since recovery techniques for a wing stall are opposite those of an ICTS. It is critical for the pilot to promptly and correctly diagnose the abnormal condition and apply the right corrective measures.

This document is from Transport Canada and discusses tailplane stall as well as a few other factors due to icing: http://www.tc.gc.ca/eng/civilaviation/s ... t-1763.htm

[photofly]

Points from the video:

The following factors require the tailplane to provide extra downwards force, and therefore fly at greater negative angle of attack, thereby risking a stall if ice-contaminated:

a) forward c of g
b) flaps extended (moves the cp rearward)
c) slower airspeed (requires up-elevator trim)

Also the extension of flaps or addition of power can suddenly change the airflow over the tailplane to a more negative AoA, causing it to become stalled.

@ dr.aero:
I can't see why any of that should get worse the faster you fly; and in fact the narrator says exactly the opposite (7:46sec). I see the connection with flap extension; but not with increasing speed.

If you look at the video of the actual stall (20:40sec) you'll see the stall happened at 84kts, and the recovery was about 90kts (after raising the flaps).

Incidentally, for single engined Cessnas the tailplane is a symmetric airfoil, and for a large portion of the cg envelope the tail is providing lift and not downforce.

[dr.aero]

photofly...

b) flaps extended (moves the cp rearward)

The reason the flaps affect tailplane stall is because they change the angle of the air that meets the leading edge of the tailplane. The cp movement rearward is a consequence of flap extension. It doesn't directly affect the angle of attack of the tailplane - which is what is of concern for stalling. There are a lot of things happening when flaps are extended. In most airplanes you've flown, what happens when you extend the flaps? The nose pitches up, not down as shown in the video. But since lift is behind the CG and assuming the airplane is in equilibrium, when the flaps are extended the lift will increase which should cause the nose to pitch down - but it doesn't. The reason it doesn't is primarily because the increase in drag (specifically with a high wing airplane), and the increase in the tailplane angle of attack which increases the tailplane negative lift and pitches the nose up, requiring the pilot to input a pitch down movement on the elevator. Once the aircraft slows down from the speed at which the flaps were extended, just like with any other configuration, the elevator will be required to move up to prevent the nose from dropping. But this has to do primarily with the flaps being extended! If they were not, this wouldn't happen because the angle of attack on the tailplane actually reduces as the airplane slows down!

c) slower airspeed (requires up-elevator trim)

Think about the angle of the air that meets the tailplane's leading edge - notice that the example at 7:46 had the flaps extended to what looks like full flap...

If those flaps weren't extended this problem most likely wouldn't happen. If you're slowing down in flight, you require more up elevator. But as you're slowing down the lift of the tailplane is decreasing requiring more elevator deflection! In the video, the flaps have put the tailplane close to its limit and then the deflection of the elevator up has increase the flow separation enough that it "sucks" the elevator down producing a violent pitch down moment.

I can't see why any of that should get worse the faster you fly; and in fact the narrator says exactly the opposite (7:46sec). I see the connection with flap extension; but not with increasing speed.

Listen at the 2:05 mark. He talks about increasing airspeed actually "intensifying a tailplane icing problem".

If you can't see how increasing speed is bad for tailplane icing it's because you don't see how increasing speed will bring the angle of attack closer to the critical angle of attack for the tailplane.

Incidentally, for single engined Cessnas the tailplane is a symmetric airfoil, and for a large portion of the cg envelope the tail is providing lift and not downforce.

Not true at all! Who told you that?

[dr.aero]

This video is of better quality: http://youtu.be/gSHjs2dhs8A

It isn't quite in sync with the other video so time references won't be exactly the same but it's close.

[dr.aero]

photofly...

I significantly edited my last reply to you.

[fish4life]

Another big part of the tail plane stall with increasing airspeed is the faster you are going the more lift the wings are producing and typically a more nose down attitude. More lift on the wings increases the down force (lift) the tail plane needs to create which is made worse since the nose down attitude tends to also increase the angle of attack on the tail plane so it's now required to create more lift (down force) at a higher AoA compounding the problem.

Apparently the Saabs were typically a tail plane stall in ice airplane which is in one theory a contributing factor to the crash of the q400 in buffalo as it is believed the CPT had reacted to the stall of the Q with his Saab tail stall training reaction which he had much more experience on. Of course we know he did a regular stall and made the problem worse in pulling back and lifting the flaps but there was more than one thing wrong with that flight that day.

[dr.aero]

photofly...

http://youtu.be/gSHjs2dhs8A?t=12m41s

They list increasing speed as one of the ways for tailplane stall to occur. They go into detail later in the video.

http://youtu.be/gSHjs2dhs8A?t=18m10s