As the prime winter icing season once again approaches, many ofus will be confronted with this sinister hazard. Every year, almostwithout fail, there are between 30 and 40 accidents involvingicing, about half of them fatal. As we’ve pointed out in previousissues, by heeding the pireps and taking decisive action at thefirst sign of ice, the icing risk is manageable, especially ifyou accept the notion that on some winter days, you’ll simplyhave to cancel your flying. The risk of serious icing will betoo great.
But what about on those gray, overcast days when ice may or maynot be present and the forecasts and pireps offer no useful information?Sure, you can always cancel when cold clouds are present or planyour flight to avoid potentially ice bearing layers, but how realisticis that? If you fly during the winter at all, sooner or later,you’ll pick up a load of ice. Maybe a lot of ice. The questionthen becomes: Now what?
In this article, we’ll examine some of the aerodynamic considerationsof flying and landing an iced up airframe. But don’t get the impressionthat I’m suggesting these techniques make it safe to fly in ice.Far from it. I’m offering these observations strictly as a survivalguide if you have to put an ice-laden airplane onto a runway someday.
The Great Unknown
Most pilots have heard this caution: When your airplane is carryingice, you’re a test pilot. If you’ve accumulated a lot of experiencein flying iced up airplanes — whether certified for known icingor not — you might not take this warning too seriously. After all,if you’ve had ice dozens or even hundreds of times and survivedit, the warning must surely be an overstatement. Maybe. But Iwouldn’t count on it.
Permit me a war story. In my flying career, I’ve been both a giverand a receiver of airframe and engine ice. Back in my Air Forcetest pilot days, around 1971, I flew the KC-135 water spray tanker,which we used to douse various airplanes to study the effect ofairframe icing. The object was to control the amount of ice build-up up on the receiver aircraft to determine its flying characteristicsand to see how well it would shed ice. Even though these testswere done under carefully controlled conditions and flown by realtest pilots, the results were sometimes unpredictable.
We had been asked to fly the spray tanker over to England, tohelp the Brits with icing certification of the Concorde. Our flighttrials were going well until about the fifth flight, when theBritish team was trying to determine the maximum amount of icethe engines could handle. We were at 16,000 feet, with the waterspray giving them a good load right into the number two enginewhen suddenly, we heard, "Uh-oh, we have a slight problemhere in the Concorde."
The engine had stalled and surged and the crew decided to shutit down. A ground inspection revealed several of the compressorguide vanes had sheared off and gone through the engine. Even25 years ago, that was a $2 million engine and I doubt if theconsortium had budgeted for that. The Brits decided they’d hadenough icing tests, thank you. They later certified the airplaneusing natural icing. The point is, the outcome of that icing testwas entirely unexpected, even though it was done under controlledconditions. If you pick up more than a trace of ice, the samemay be true for your airplane.
Obviously, the best way to avoid an unpredictable outcome is tostay out of ice in the first place. When the pireps confirm thatit’s widely present, stay home, drive or go commercial if youronly other choice is to fly an unprotected airplane. If you doencounter ice that continues to accumulate, don’t hang aroundwaiting for it to stop accreting. Formulate a plan right now.A couple of years ago, when we reviewed 170 icing accidents foran article, we found that many pilots underestimated both therate of accretion and how it would affect aircraft performance.
In more than a few of these accidents, pilots reported icing toATC then declined to divert or declare an emergency until it wastoo late. The accident data strongly suggests that once ice hasaccumulated to the point that the airplane will no longer maintainaltitude, the chances of making it safely to an on-airport landingare poor. Given that the majority of icing accidents seem toinvolve experienced pilots, it’s reasonable to assume that pilotsfall into the trap of concluding that one icing event is justlike the next. The facts suggest otherwise. Ice — and its effectson airframe and engine — is extremely variable. Just because you’vesurvived 99 icing events, doesn’t mean you’ll survive the next.Resist the instinct to tell the controller you don’t have a problem.If you’ve got ice, you’ve got a problem.
Drag and AOA
Even pilots with lots of experience flying in ice don’t alwaysunderstand the aerodynamic penalties of hauling around a loadof it. Ice adds both weight and, more significantly, tremendousdrag; cleaner airfoils on high performance airplanes may be moreefficient collectors of ice and will suffer more from its effects.
Attaching meaningful numbers to the damage ice does to lift anddrag is difficult, since it varies with airplane and airfoil.However, icing research done by Dennis Newton and reported inhis excellent book Severe Weather Flying, revealed that typically,even a small buildup can reduce the maximum coefficient of liftby 30 percent. This has the effect of decreasing the stall angleof attack, which translates to a higher stall speed. Newton saysthat even a 1/8-inch buildup raised the clean stall speed of oneairplane from 69 knots to 80 knots. A further accretion of 1 1/4inches increased the stall speed only another 4 knots.
Drag keeps on building up with further accretion, however, andthis is what paints many a pilot into a corner from which thereis no exit. As drag increases with a continual buildup of ice,the power required to maintain cruise airspeed or even an airspeedabove the stall also increases, to the point that even full throttlewon’t do the job. If this happens, the pilot has no choice butto enter an involuntary descent and hope for the best. Furtheraggravating the angle-of-attack/drag issue is the fact that inicing conditions, the engine probably won’t be capable of deliveringrated power. In a carburated engine, carb heat should be on inicing. But the warm air going into the induction system is lessdense and reduces the engine output. Fuel injected engines withmanual or suck-open alternate air intakes generally draw air frominside the engine compartment and it too is warmer than ambient,resulting in somewhat lower power yield. Ice accumulation onthe propeller is no help, either. When these factors are takentogether, even a moderate accumulation on an underpowered airplanesuch as Cessna 172 or a Cherokee can soon result in the inabilityto hold altitude.
A note on prop de-ice: The theory that a clean prop will haularound a lot of ice, even if the wings aren’t protected by boots, is a dangerous misconception. Newton’s data revealed that powerloss from iced props amounts to about 9 percent, but is rarelymore than 20 percent. The additional power required to move aniced wing through the air, with its higher drag, may be as muchas 250 percent.
It’s long been known that small radius or sharp-edged surfacesare more efficient collectors of ice than are blunt or large radiusobjects, such as the wing’s leading edge. That’s why the firstplace to look for ice is on a sharp-edged projection, such asan OAT probe or a protruding fuel vent. It’s also the reason thatthe tailplane’s relatively small leading edge may collect icetwice as fast as the wing. The consequences of this have beenunderstood only recently, following a spate of accidents in whichtailplane stalls were suspect.
To understand the tailplane stall, recall your private pilot groundschool sessions in which flight mechanics were discussed. Thecenter of lift of the main wing is such that a stable aircrafthas a natural nosedown tendency. The horizontal stabilizer’s jobis to counter this, by exerting downward lift and thus noseupmoment. As does the main wing, the tailplane has angle-of-attacklimitations, but in reverse to those that apply to the wing. This means that if the tailplane stalls, it essentially stops makingthe downward lift needed to counter the nosedown moment. Result:The aircraft may pitch down uncontrollably.
Knowing that ice effectively lowers the stall angle of attackon the main wing, it’s easy to see how this works on the tail,too. But since the tail is a more efficient collector of ice,it may reach the stall angle-of-attack before the wing catchesenough to be a problem to the pilot. In fact, it’s conceivablethat the tailplane could have enough ice to stall it, when none is present on the wings.
Flaps aggravate the problem. In all airplanes, flap deploymentproduces downwash that affects the tailplane and elevator. Insome airplanes, this downwash is so pronounced that it producesa local airflow that changes the tailplane angle of attack substantially.Ice acts like a stall fence, encouraging the onset of tailplanestall. Tailplane stalls have gotten a lot of press since 1991,when NASA and the FAA conducted a conference on the topic of tailplaneicing. Since then, several accidents have been directly linkedto tailplane stalls and, retrospectively, other heretofore unexplainedaccidents may also have been the result of tailplane stalls. Mostrecently, the NTSB investigated two accidents involving BritishAerospace Jetstream turboprops in which tailplane icing was suspect.A third Jetsteam accident in December 1993 took placein icing conditions but, in the final analysis, ice wasn’t considereda factor. Other airplanes, including the ATR 42, Saab SF340A andEmbraer EMB-100 have been the subject of ADs having to do withtailplane sensitivity in icing conditions.
So much for turboprops. What about the Cherokee and Bonanza crowd?Are these airplanes as susceptible to the hazards of tailplaneicing as are the turboprops? Do any light airplanes have a particularlynasty history of tailplane stall incidents and accidents? Theshort answer is we don’t know. Nothing excuses light singles andtwins from the same aerodynamics that apply to turboprops butwe are unaware of any body of research that shows that any ofthe 30 or 40 icing accidents each year are due to tailplane stalls.
Our review of four years’ worth of icing accidents (1974-75,1984-85) revealed that about half were fatal crashes. There’snothing typical about fatal icing accidents, other than a pilotencountering more ice than he or the airplane could handle. Mostof the fatals do result in uncontrolled flight into terrain butwho can say if these were the result of tailplane stalls, wingstalls and spins or some other factor? Unless the pilot survivesto tell the tale, accident investigators don’t have a lot to goon and they’re generally reluctant to speculate.
Not too surprisingly, a twin is more likely to make it to a runwayafter a severe icing incident than a single is. But once overthe threshold, a landing hard enough to damage the airplane isoften the result. Many of these pilots reported being unable toflare, even to the extent of pitching to a level attitude. Lackof noseup pitch authority or very heavy elevator forces are consistentwith a tailplane stall. But again, given the paucity of data inthe accident reports, that’s but an informed guess.
Approach and Landing
So how do you translate these aerodynamic basics into a survivalstrategy for approach and landing? The important thing to rememberis this: With ice on an airframe never certified to fly in it,you really are a test pilot.
First, I’ll assume you’ve accumulated ice and, rather than tryingto get out of it, you’ve decided to divert and land ASAP. That’susually a good choice and one that should be made sooner ratherthan later. The longer you wait, the fewer options you’ll have. If you even suspect a diversion may be necessary, start planningfor it and have plates at the ready. Know what the weather isalong the route and keep up with it by listening to ATIS reportsand/or checking with FSS regularly. (While you’re at it, passalong any icing — or lack thereof — pireps for your fellow pilots.)
Tell the controller right away what your plans are. If the controllerasks "are you declaring an emergency?" think very hardabout your answer. By declaring, you’ll be given priority andyou’ll get out of the ice that much sooner. If you decline, thecontroller will handle you first-come, first-serve, and that maymean a roundabout vector and more ice. As we’ve said so many timesbefore, it’s a myth that declaring an emergency results in a lotof paperwork or regulatory hassles.
If you have a choice, pick an airport with a good approach, preferablyan ILS with approach lighting. Having accurate course and verticalguidance will be a real help, especially if the windshield isiced over and difficult to see through. Furthermore, an ILS runwaywill usually be 5000 feet or longer, enough length to land withminimum or no flaps, which is the recommended procedure.
As far as descending for the approach is concerned, that willtake some judgment and what you want to do may not square exactlywith how ATC wants to run the program. Depending on how the airspaceis set up, the controller’s normal procedure might descend youto 2000 AGL on a vector 10 or 12 miles from the airport. But ifthe ice is still building and the freezing level is right to thesurface, maybe you shouldn’t give away that altitude so far out.
This is doubly true if you can barely hold altitude going intothe approach in the first place. Consider joining the approachhigher than normal and descending on or slightly above the glideslope,in a steady, controlled descent with power on. In any case, evenif you’re visual, don’t execute a long, dragged in approach withperiodic level-offs. You may find out too late that you don’thave the power to arrest the descent.
On the other hand, don’t try or accept a slam dunk. A brisk descentthrough an ice-bearing layer is one thing, but a banshee divefor a lower altitude invites an abrupt pull out and increasesthe chances of descending right through the MDA. That’s apparentlywhat did in the crew of the Express II Jetstream in Hibbing, Minnesotain December, 1993.
Speed and Configuration
How fast to fly the approach? Those with expertise in the fieldmay not agree on the exact numbers but everyone agrees on this:fly it faster than normal. Dennis Newton recommends a 20 percentincrease, my own guideline is 10 to 20 knots faster. Even witha tiny coating of ice, you won’t know what the stall speed isbut if you fly the approach normally, you may learn the hard waythat it’s much higher than you imagined.
If 20 knots extra is good, won’t 40 knots be better? Perhaps not,for two reasons: First, you’ve got to land out of that approach.Crossing the threshold 40 or 50 knots fast with flaps up willeat up a lot of runway. And if there’s ice on the airplane, therunway may be icy, too, or at least wet, further complicatingyour ability to stop. Second point: The higher the speed, thegreater the risk of a tailplane stall. At the higher speed, themain wing angle of attack is lower, meaning the tailplane willhave a higher negative angle-of-attack. And like the main wing,if the tailplane has ice on it, it will stall at a higher angleof attack.
As for flaps, again no universal recommendations, other than usingfull flaps is a bad idea. Full flaps aggravate the tailplane stallsituation and they may require a much larger pitch moment to roundout and flare for landing. Besides, if you really don’t need them,they’re just another surface to collect ice. Very few singlesare certified for known icing, so chances are the manufacturerwon’t have flown the aircraft with iced wings in any configuration.You’re on your own.
Newton cautions against using any flap setting you haven’t firsttried with some altitude beneath you. Since it may not be convenientto conduct any testing while in an icy stratus layer, that mightmean landing with no flaps. So be it. There aren’t many singlesthat you can’t land safely with no flaps and this probably appliesto a lot of piston twins, too. Next time you’re practicing approaches,add flapless landings to your routine, so you’ll know what toexpect. Aircraft that are icing certified may or may not haveflap recommendations. If yours does, follow the book. If it doesn’t,use no flaps or no more than half flaps. Gear can be extendednormally, unless you’re worried that extending it too soon willproduce an unarrestable descent rate. If that’s the case, you’llhave a difficult choice. Extend it and hope you make the runwaybefore settling short or leave it in the wheels until crossingthe threshold or simply plan to land gear up. That may be a betterchoice than plowing into the approach lights short of the runway.
The Home Stretch
Our review of icing accidents showed that quite a few airplanes,especially twins, make it over the threshold safely, only to sufferdamage as the result of a hard landing. As I said, this couldbe due to tailplane stalls, wing stalls or failure to see wellenough through an iced windshield to arrest the approach descent.
To avoid this, keep the power on and the airspeed up right acrossthe threshold to the runway. Fly the airplane onto the runwaywith only enough roundout to arrest the descent. Don’t worry abouta two-point touchdown, a greaser or, least of all, a full-stalllanding. Flying an airplane onto the runway with no flaps willfeel fast but if you maintain directional control while you decelerate,you should have plenty of room to stop.
If you go visual well out from the airport, pick the longest runway,wind permitting. But be very, very careful about circling closein. With iced wings and a higher stall speed, even a moderatelybanked turn could stall one wing, resulting in loss of controlat low altitude. Keep the turns shallow.
As you plan your flights this winter, remember that icing is rarelya bolt-from-the-blue surprise. It’s forecast more often than it’spresent but when it is there, it’s usually expected. An inadvertentencounter with ice needn’t been the gut-wrenching terror we imagineit to be. Just have a plan of attack and be ready to exerciseit and you’ll come out just fine.