lzakplt

Just thinking out loud here, maybe the presence of glycol on the tail is a contributing factor.

Cubdriver

As mentioned in the other thread there very well may be ice-glycol buildups within the hinge gaps near elevators that can inhibit movement. Only a deliberate check for them by moving the elevator will find them. I have never seen them myself, but they are supposed to be big enough to stop the flight surfaces from moving. The airplane would have had to have flown recently and been deiced recently for this to happen. It happens when the residue from previous de-icings gathers in crevices and refreezes by admixing with moisture from clouds. I am not sure how this could play into the uncommanded pitching up problem.

lzakplt

That is not the direction I was thinking. The Caravan POH has a specific t/o procedure when type II or IV anti-ice fluid has been applied. Use no flaps, and rotate and climb faster than normal. As far as I know, this is a pretty normal procedure for any low speed a/c which has had anti-ice fluid applied. Anti-ice fluid is more viscous than de-ice fluid, and needs high airspeed in order to blow it off of the plane. The special procedure is designed to blow the anti-ice fluid off of the plane before it is trying to fly.

I am aware anti-ice fluid is different than the de-ice fluids we routinely use. De-ice fluid has lower viscosity and isn't supposed to need extra airspeed to blow it off prior to t/o. Having said that, it is not unusual to get de-iced, fly for 40 minutes and still find de-ice fluid residue on some a/c surfaces.

What I am supposing here is a possibility that a small amount of de-ice fluid adhering to the horizontal stabilizer at the point in time when tail force should be switching from down to up is enough to contribute to airflow separation under some circumstances. I think this possibility is all the more plausible when one considers that airflow patterns over the Caravan's tail change radically when tail forces are shifting from down to up. During initial climb out when the tail is producing down force perhaps there are spots on top of the horizontal stabilizer from which the fluid does not blow right off?

Again, I'm not saying de-ice fluid is interfering with tail section airflow, I'm just considering the possibility.

185flier

IZA
that actually makes sense to me

something is causing a higher than normal accident attrition to this aircraft in the winter months
no one can deny that
what is it
i don't believe cessna that it is all the pilots fault for flying in ice
there is more to it than that or we would see in increase in the accidents for all types due to pilot inadequacy
cessna does not want you to believe this

think about it

185flier

anyone thinking

TangoBar

I don't believe for a minute that Cessna designed, or allowed to enter into service, an aircraft whose horizontal stabilizer 'switches' from providing downforce at the tail to lift at the tail at any point during flight. Why? Because for that to happen, it would either result from or require a MASSIVE change in the CG. The downforce required is the same at liftoff, climb, cruise, and approach speed- remember that the fuselage acts as a see-saw suspended from the Center of Lift. Downforce is not a function of airspeed, it's a function of masses and the Center of Lift. If you have to REMOVE downforce from the tail end, it means you've REMOVED the load at the other end. You can't just say that it switches without thinking about why you had the downforce in the first place. If there's evidence from a deposition, can you post a link to the information, or at least a reference to the particular case where the engineer made this claim?

Look, a Caravan is built like a big 172 or 182. Neither of those a/c allow the CG to get aft of the Center of Lift. NO Cessna, Piper, Beech, Cirrus, etc., is designed that way. If any a/c were designed to 'switch' in the manner suggested, it would be so sensitive to pitch force changes that it would be virtually uncontrollable.

Now, as for the different procedure for the de-ice fluid? I'm willing to bet that rather than trying to blow the fluid off the plane, the revised procedures take into account the fact that a high-viscosity fluid coating the surface can disrupt airflow as it ripples and flows, and the revised procedures are designed to modify speeds/angles of attack to counteract the change in surface characteristics caused by the presence of the heavier fluids.

Cubdriver

Correct, CG location does not move very much if at all. The center of lift for the airplane does however and it moves forward in this case due to the shorter chordline created as flaps retract back into the wing. Caravan has oversize fowler flaps for meeting the 61-kt stall speed requirement. As a result, spoilers are required to augment the undersized ailerons. Retraction of the flaps may put the location of the center of lift forward enough to be forward of the CG location and an upward tail force would be required to counterbalance it.

FAA certifies an airplane for a type certificate only after rigorous theoretical and practical analyses have been performed. Aircraft manufacturers are held to a high standard in this country. That doesn't mean a problem cannot occur, but that in all the relevant areas of construction, operation, performance and theory the airplane has been examined and found able to pass by specific margins. Longitudinal stability is one of the areas it must pass, both in theory and in testing.

It's not so simple. Dynamic pressure on the tail always changes with changes in airspeed, prop wash, propeller slipstream, downwash from the main wing, and sideslip if there is some. The tail force increases and decreases and may change dramatically as the steady-state flight configuration goes from climb to level flight to descent. CG is only part of the total longitudinal static stability equation. The latter is a large equation with a ton of greek letters and is beyond the scope of our hangar-style discussion. CG may shift as fuel is burned off also, but not much in the Caravan.

185flier posted some hand calcs he got somewhere, but they are a bit hard to use without clarification. The real thing to have would be the flight test data from original certification which is not public information.

Center of lift can be summed across the entire airplane. If the addition of the main wing and the tail produces a combined center of lift aft of the CG there is positive static stability. The main wing need not be designed with a center of lift aft of CG. Usually a tail will have a down force on it to create a nose up moment which opposes a nose down moment created by a wing, but not always.
Actually it would go to neutral stability before negative stability. The airplane will no longer correct back toward a moderate angle of attack, roll or yaw, and will stay wherever last put. This is not desirable for normal and utility category airplanes, because if it goes unnoticed it may end up somewhere away from what the pilot intended. It is not a recipe for disaster necessarily and aerobatic airplanes have neutral static stability on one or more axes. For example, aerobatic airplanes do not have dihedral usually so they tend to stay where placed in bank angle.

Some of the de-icing are fluids intended to stay stuck to the surface in flight. They have a negligible effect on boundary layer adhesion.

lzakplt

Here is what one manufacturer of anti-ice fluid says on the matter:

"Type IV fluids contain proprietary thickeners to enable the fluid to form a protective film on treated surfaces, providing holdover protection against ice formation and/or snow accumulation, yet shearing off the aircraft wing upon takeoff."


"The thickeners in SAFEWING MP IV 2001 anti-icing fluid are designed to release or "shear-off" the wing during takeoff (before rotation)."

http://www.lyondell.com/Lyondell/Pro...ti-icingFluid/

185flier

I got the calculations from one of the officers of PenAir
they are calculations of their lifting moments and balance of the accident aircraft of 2001
they were computed by Dr. Orloff who is an aerodynamicst and show the lifting requirements of the plane
I wish I could explain them

185flier

CESSNA 208 CARAVAN NEEDS CRITICAL "KNOWN ICING" IMPROVEMENTS FOR SAFETY

The Cessna 208 is a marvelous airplane for

carrying lots of people and heavy cargo, but only in good weather. Flown in icing conditions the airplane is dangerous and has crashed thirty times, and nearly crashed many more. The NTSB has designated curing its dangerous history of accidents Public Enemy Number One. Nine people have been killed this icing season so far, and it's only half over.
We have represented several families whose lives have forever been changed because of the Caravan's poor performance in even light icing conditions, conditions for which Cessna Aircraft Company (the plane’s maker), and BF Goodrich (the designer and maker of the deicing boots) have said the airplane is suitable. In fact, it is not. The Caravan should not be flown in any icing conditions, and some of the operators refuse to dispatch it into any known icing weather.

The problems with the Caravan are simple. It is underpowered and, in fact, it appears to have the lowest power to weight of any turboprop single. That means it cannot climb above the ice and thus avoid it before its aerodynamics are so compromised it suffers a drastic loss of performance and control.

It has too much parasite drag, meaning there are so many unprotected surfaces that when ice collects, it seriously and quickly degrades performance to dangerously inadequate levels.

It has deicing boots that are simply inadequate to protect the wings and tail so dangerous ice accumulations even when the boots are used properly, quickly and dangerously compromise control and thus safe flight. Often there is insufficient margins to exit icing conditions safely and climbing may be impossible due to low power.

The Caravan uses engine bleed air to operate the cabin heat and the boots. It has no separate pump to operate boots like some other turboprop airplanes and has no water separator to keep moisture that collects in the boot inflation tubes from freezing and compromising the boot inflation. The bleed air extraction, together with the loss of power from deployment of the inertial separator designed to keep ice from damaging the engine compressor, drastically reduces the already underpowered airplane's ability to exit icing conditions.

The aerodynamics of the Caravan also play a role in its inability to safely handle ice. Its horizontal stabilizer does not have ice protection to the tip and the elevator balance horn is entirely unprotected. The tail provides an up force, unlike most others that provide a down force. Thus, the top surface of the horizontal stabilizer is critical. Ice on this surface causes a pitch up, loss of airspeed, wing stall, tail stall, and drives the center of lift on the wing so far aft that regaining control at any airspeed is questionable.

The failure of the FAA to understand the aerodynamics of this airplane is unforgivable given the repeated concerns expressed by the NTSB and its own knowledge of the problems reported by pilots in Caravan winter operations.

Recent Safety Recommendations by the NTSB about the Caravan bring credit on that agency's understanding that "something is wrong here" and that previous blame on pilots for accidents beyond their control is unfounded.

The Caravan is fundamentally a good design for fair weather flying. It should have had anti-icing equipment, not deicing boots that by design allow dangerous amounts of ice to collect before shedding and leave lots of ice as a residual of their operation. The Caravan should have had a cantilevered wing, instead of drag producing struts and, if cargo pod equipped it desperately needed, pod anti-ice protection. The powerplant is in need of twice the horsepower for this mission, and elevator balance horn anti-ice protection is vital.

The Caravan can be fixed and, if it had been fixed when the FAA first started to investigate icing incidents and accidents shortly after the aircraft was introduced, the airplane might have been well suited today for the all weather operations it is touted by Cessna to be capable. It is not, and the Randolph, Fry and Silvey families have suffered horribly, as have then ten little girls who have been left fatherless.

The FAA and the NTSB must do better. Twenty years have gone by since the first investigations and still no positive and effective efforts to fix the airplane. It is clear that at least the FAA lacks the technical expertise or will to understand the aerodynamics of the Caravan. At this late date, after three separate safety investigations, the FAA still thinks the bottom surface of the horizontal stabilizer is the critical lifting surface. It isn't!

It is also unfortunate for Cessna, who has had ample opportunity to fix the airplane, yet still denies it has a problem. This is litigation driven no doubt so, instead of fixing it and avoiding other accidents, other tragedies for the victims' families and other lawsuits for wrongful death, it denies the problem that everyone, including the federal authorities, knows about, and allows more accidents and more claims. Aside from the moral bankruptcy of such a position, from a purely economic standpoint, it is inexplicable.

There is hope, however. Others recognizing the problem have started addressing it themselves. Weeping wing TKS retrofits are now available to provide anti-ice protection. Larger and more capable powerplants are being STC'd by others for the aircraft and even a hot wing anti-ice system is being tested. Hopefully these non-Cessna designed and built modifications will save lives, but must be purchased at substantial cost by operators of these aircraft.

The icing accidents and incidents involving the Caravan have reached intolerable levels. Something must be done and done quickly if others are to be saved. After nearly forty years litigating airplane crashes, it never ceases to amaze me that aircraft manufacturers won't listen. Airplanes always telegraph their intention to fail long before they suffer a fatal accident. Fixing the airplane before the first accident is the least expensive means to reduce the cost of air crash litigation liability, and fixing it after the first accident will guarantee that there will never be a claim for the same defect after the payment of the first one.

Icing Season 2005/2006