Cessna Caravan icing problems
 

Hello to you,

As a part of the theory education at my flight school (SAA in Sweden) we are to do a project about something related to aviation. The issue I have decided to investigate is the Cessna Caravan icing problems. I first came across it when reading an article about the Caravan in some magazine and a visit to aviation-safety.net confirmed, indeed there were quite some accidents involving the Caravan and many of them related to icing.

I can see a couple of factors why the Caravan is accident prone.
-The Caravan itself sensitive to any ice accumulated.
-The Commander making 'bad' decisions to take-off into icing conditions. Perhaps stressed or anxious about having to perform a flight even though the weather is marginal.
-The Operator, many accidents seem to strike contractors to majors like FedEx. Do they provide essential training on the Caravan in icing conditions? Contributing, many of them seem to operate the Caravan single pilot.
-The area where many accidents have encountered is north of America and Canada, areas with mountanious terrain, fronts, lows etc.

I fancy the question at issue in a manner it involves both Principles of Flight and Human Performance. :8

The reason I am writing here is perhaps I could find a real Caravan pilot to share any ideas (if there are any to share?). I am not familiar with the Caravan aerodynamic characteristics, only the basic on how ice affect the performance. Higher stall speed, more drag etc.

Anyone involved in this kind of operation (as described above) or previously involved might have any furhter comments to add as to why the Caravan is so unlucky.

Thanks in advance for your help in this matter! :ok:

/Patrik

the Cessna Caravan is certified for known icing?

that is a tough certification---If these aforementioned icing incidents are ---continued flights into known icing--- then accidents are inevitable weight increases, drag increases, lift decreases, more power required ---welcome to test pilot school!

if it IS certified then hopefully airworthiness directives have been written and applied or the deicing certification will be downgraded or totally decertified---this seem like an interesting topics I have interest in part 23 icing certification.

Patrik,

All aircraft are sensitive to icing. Cessna produced pilot notices several years ago addressing the need to maintain speed and restrict flap use in ice; these cautions applly to all high wing Cessna's, particularly the 200 series airplanes.

Avoiding ice is always the right choice. A number of accidents have occured when Caravans flew into icing conditions, but then accidents have occured in many aircraft types due to ice.

The nature of ice is that it can be highly unpredictable. I've been in ice when the aircraft ahead was reporting no ice, the aircraft behind was reporting no ice, and I had severe icing. In that particular case, it was very localized due to orographic, or terrain-induced lifting. However, I've done a certain amount of icing research in flight, including carrying some sophisticated sensor equipment aloft to measure ice crystals and water droplets as part of atmospheric testing. I've seen icing be virtually non-existant one moment, then blossom into a rapidly building situation the next. Remember that the definition of severe icing is any time icing exceeds the ability of the aircraft systems to control and remove it...what's mild ice to one airplane can become severe to another.

If an airplane is iced and can't remove it, then the best that can be hoped for is that the aircraft will continue to fly safely with the amount of ice it's carrying. That's the best that can be hoped for. It only gets worse from there, and if the aircraft isn't doing well presently, any more ice will only make things worse.

Tailplane stalls have been thought to be applicable to T-tail airplanes, but other aircraft are fully susceptible, and the Caravan is certainly among them. the horizontal stabilizer is thinner than the wing, builds ice more easily, and can stall, resulting in nose own pitching moments...requiring pilot actions opposite to what one might normally take in a conventional stall condition. These pilot actions include decreasing engine power, unconventional flap practices (raising or leaving alone, if the stall occured when flaps were applied), and applying aft elevator to increase elevator download. When the stall occurs and download is lost, the pilot may be unable to prevent the aircraft from pitching nose down. If this occurs close to the ground, recovery may not be possible.

This applies to many aircraft other than the Caravan, of course, but the caravan has seldom been thought of as a tailplane stalling airplane, when in fact it is.

An important aspect of ice to consider is that while the airplane can carry a very heavy load of icing, it's not particularly tolerant of the changes in aerodynamics; particularly disruption of the airflow layer over the top of the wing, and changes in the angle of the downflow at the back of the wing.

I've hung out of Caravans on many occasions while skydiving, often with a whole group of people standing on the side of the airplane, hanging off, prior to letting go for the skydive, and the airplane is tolerant to a point. I've also flown them and run out of control when enough people are hanging on outside the airplane, horsing around.

Even a small layer of frost on the wing is enough to seriously disrupt lift, causing early boundary separation on top of the wing, and resulting in significant performance loss.

Interesting fact you're presenting about your own testing. Perhaps the reason why the pilots of Russian Caravan that went down talked about everything about no ice to severe ice.

The feature 'similar threads' was really helpful and I found some pretty good info in those. This one for instance: http://www.tsb.gc.ca/en/media/communiques/air/2006/comm_a05c0187.asp

Regarding the operation of de-ice boots. I have no experience myself but I try to recall from the ASP class. If I am not mistaking de-ice boots shall not be operated at once ice is building up on the wing but after a layer (how thick?) of ice has built up - otherwise the ice will not crack and fall off. I don't quite understand this to be honest, if the layer isn't deep enough to crack completely during the first cycle it might crack during second or third cycle of the boots. Is this something they pay extra focus to during thechnical courses?

The phenomenon you're talking about is ice bridging, in which early application of deice boot pressure will inflate the boot, pushing soft ice outward, but not breaking it off. According to ice bridging proponents, the ice then remains in the shape of the inflated boot, and continues to build. The boot deflates, leaving a hollow channel beneath the ice. Each subsequent inflation of the boot fails to affect the ice, as it's built a "bridge" over the boot...the boot can no longer properly remove the ice.

In the USA, the Federal Aviation Administration and NASA have both come out in recent times and adamantly insisted that ice bridging is a myth, that it does not exist.

It does.

What the FAA is pushing is activation of the boots at the first sign of ice. Their stand, and also that of NASA, is that failure to activate the boots early in the icing process may prevent the boots from doing their job, too...too much ice and the boot won't work, either. They also don't want the ice to build up over the boots and then start building aft of them in the unprotected areas of the flying surface; when the boot does break away the ice over the boot, it leaves a ridge behind the boot which disrupts airflow. Take care of it early and keep inflating those boots, says the FAA.

I disagree, and most experienced aviators who have spent any significant time in icing conditions with boots will know that ice bridging isn't a myth...because we've seen it. Time and time again.

Some suggest waiting until a quarter inch or a half inch of ice has built up, and I disagree with that, too. Ice builds more readily on ice than it does on a rubber boot, and if you let ice build and form over the boot, the rate of accretion , or the rate at which the ice builds, will be greater; more sticks, and it sticks better than it does over the airframe. This is especially true of rime ice, which can build at fantastic rates once it gets started.

Personaly, I advocate letting experience be your guide. If you have soft ice out there and pop or blow the boots too soon, it will bridge, and then it's hard to get off. What is needed is letting just enough ice build that it can be properly broken, and the ability to judge this comes with time. I also advocate staying out of ice where ever possible, and if you do get in ice, get out of it as soon as you can. Very often a simple altitude change will do it, if you make the change soon enough.

I've seen ice build so rapidly that the aircraft loses it's ability to make those changs. I experienced that in a Twin Commander once, in which ice built rapidly enough that 50 knots of airspeed was lost in less than a minute, and altitude could no longer be maintained. We had no choice but to descend, and did until we made contact with a road, and followed the road to a local airfield. It was a very uncomfortable experience, made worse as we were in a mountainous location with severe limitations on our ability to divert, or turn, and once the icing began we could no longer make the minimum enroute altitude. It happened very quickly, and started next to a sharp rise in terrain which pushed a lot of moisture into the freezing level. Our systems couldn't keep up with it.

A case like that calls for immediate agressive activation of all anti-ice and de-ice, and immediate action for diversion, altitude changes, etc. It's very serious.

On that note, you'll study of course the differences between anti-ice (preventing ice) and de-ice (removing ice)...but should understand that it generally works best when applied before entering icing conditions (boots excepted, as they don't do a thing until ice builds, except slightly disrupt the aerodynamic shape of the wing or horizontal stab, or inlet). Hot sufaces must be hot before you enter the ice, and if you're waiting until you're in the ice to turn them on, it may already be too late. This is especially true of stall and pitot anti-ice, inlet anti-ice etc. Turn it on early and keep it on until you're sure you're not in the ice.

Icing occurs above freezing, and continues down to about minus ten to minus 15 celsius. That's the ideal range. Icing can occur above freezing ambient temperatures due both to inaccurate temperature sensors, and from freezing rain and other phenomenae. It can occur due to local drops in temperature around the airframe where airflow is accelerated, pressure drops, and ice forms. (Carburetor icing is a good example). Icing can also occur at very cool temperatures...liquid water, in the form of supercooled water droplets, has been encountered right down to minus 40 degrees. It does happen. In any event, having your heats on and your ice protection active before encountering ice is the best policy...any time you're in icing condtions.

Icing conditions are any conditions that might cause ice, are conducive to ice, are producing ice, in which ice is forecast, in which is is reported, in which ice is possible...even when it's not being reported or forecast. Ice can easily form when it hasn't been forecast. Ice can for when others aren't reporting it...after all, someone has to be the first to experience it...it might just be you.

There's a lot of material out there on ice, and for all the time and experience the industry has with ice, there's a lot more yet to be learned. You certainly live in the right part of the world (Sweden, is it?) for that...learn as much as you can about it, and always assume there's more to learn as you go. Good luck!

SNS3Guppy, thanks a lot for your effort writing it down!!! Interesting for a young chap and wannabe pilot like me. :D
Last year I sat as dispatcher at the flight school struggling with rostering for the instructors, just to witness all flights cancelled day after day after day during the winter months. Icing in clouds and the BE76 is not de-ice nor anti-ice equipped.

A wise choice; flights can always be made another day.

I've done all kinds of urgent flying, including firefighting and emergency medical (EMS). One thing I've learned over the years, or rather decided based on my own experiences is that there is no flight which must be made. None.

Knowing when to say know, knowing when to say yes, and knowing the difference between the two is the essence of what flying is about. We don't get paid to drive airplanes. A monkey could be taught to do that. We get paid to make decisions, and to stick by them when we know they are right.

Caravan Accidents
 

Go to the NTSB.gov website and research the 208 accidents. The one where icing may have had a part in the accident, look closely at the reasons. All of them happened due to the pilot operating the plane beyond the limitations of the plane. Pelee Island Caravan accident is a great example, the pilot took off 15 percent over gross weight with ice on the plane and into severe icing conditions. Moscow accident, in severe ice with the autopilot on and never pushed the Power Lever up after the autopilot leveled off at 5000 feet, just stupid stuff like that. I could go on but you look them up and decide. Winnipeg accident, the pilot took off over gross weight into freezing rain with contamination on the wings. Look at the special weather release over the ATIS, 15 min B4 takeoff. Temp -2 and FRZ DRZ. Look for yourself. Then on top of that the pilot never got to minimum speed of 105 KIAS ever. 105 was the min spd in icing and is a limitation on the plane. The Moscow plane was below 105 also. Take Care

Does anyone know if some of the Caravan icing accidents have been due to tailplane stall? From the reports I've read I could not determine whether it was ice induced wing stalls or tailplane stalls that made some of the naircraft fall out of the sky. Recovery procedures are vastly different.

What I am trying to find out is whether tailplane stalls are realistic reasons why some Caravans have crashed or if wing stalls are more likely in this case?

Perhaps the answer is obvious, if so excuse my ignorance.

I've read 5 accident reports regarding the Caravans, of which three never left the ground. All winter time flight related and pilot error. The other 2 did involve pilot error when landing. However, there were no injuries/minor in any of these events.

1) [overweight icing on takeoff] http://www.ntsb.gov/ntsb/brief.asp?e...12X22480&key=1
2) [lack (seasonal/rain/fog) of vision on descent] http://www.ntsb.gov/ntsb/brief.asp?e...12X22063&key=1
3) [winter weather] http://www.ntsb.gov/ntsb/brief.asp?e...03X00015&key=1
4) [snow covered rwy w/tailwind] http://www.ntsb.gov/ntsb/brief.asp?e...06X00190&key=1
5) [hard landing/icing/inadequate flare/poor forward visibilty] http://www.ntsb.gov/ntsb/brief.asp?e...15X01241&key=1

Cloud Seeder
 

SNS3Guppy,

Many moons ago I did a lot of cloud seeding, both line seeding at minus 5 degree centigrade and cumulus/cu nim in a Cessna 310 with minimal de ice/anti ice equipment. In the pre-flight met briefing for the area to be seeded, it was most important to know the outside air temperature at the lowest safe altitude. If the OAT at Lsalt was not above zero, then the line would not be seeded. Our location of seeding was in Australia, so the system worked well. In other countries this would not be the case.

In line seeding the aircraft would be flown until the ice build up caused the wing to approach the stall (when 5 degrees of bank would cause pre-stall buffet),then the aircraft would be put in the descent at about 140 Kts Ias until the ice broke off in sheets, then back up to seeding altitude and do it all again. This system worked well and taught a lot about controlling an aircraft close to the stall.

The value of the unsophisticated, simple, outside air temperature gauge made the operation a success. In an emergency it can be used to replace an altimeter indication.

Tmb

The Dllingham accident in 2001 seems to be related to tailplane stall. Take-off with ice remaining on the airframe. During climb out the nose pitched up suddenly followed by pitch down and crash.

Interestingly some people teach that the tailplane produces an upload on the Caravan, unlike most aircrafts, and that seems to be valid in this case. Upload or download, could that depend on the CG position for each flight and what is the normal configuration?

Dillingham Accident
 

A jury in Anchorage, Alaska, reached a verdict in favor of Cessna in a lawsuit arising from the Oct. 10, 2001 crash of a PenAir Caravan near Dillingham, Alaska. The plaintiffs, relatives of the 10 people killed in the crash, claimed the Caravan had design defects that made it dangerous to fly in icing conditions. The jury found that “no defects” of the Caravan contributed to the accident. Cessna said, “Again, we wish to extend our heartfelt sympathy to those who lost family or friends in this accident. As to the verdict, after weighing the evidence, the jury has reached a conclusion that, in fact, coincides with what we have known about the Cessna 208 since the first one entered service in 1985. It is well designed and safe when flown within the parameters of the pilots operating handbook.” The NTSB concluded in January 2003 that the probable cause of the accident was “an in-flight loss of control resulting from upper surface ice contamination that the pilot-in-command failed to detect during his preflight inspection of the airplane.”

There is too much flight time and Flight tests by several agencies to have a tail that stalls for no reason.

So one would think that the tail only has an upload during the winter in visible moisture and cold. Do you think the tail upload stall has been masked by these accidents in cold weather, so by circular logic, the tail only has an upload during the winter in visible moisture.

Where in the heck is the tail plane upload stalls in normal weather?You have to show that the critical angle of attack on the tail is exceeded for it to stall.

Reviving an old thread here.

I have a question as to how much better the TKS system is on the Caravan compared to the boots. I understand the difference and that it should be better, just looking for comments from anyone who has real experience as where I fly is looking to buy one.