Every year, like clockwork, widespread icing conditions force pilots to take a hard look at how they analyze weather, weigh risk and exercise judgment. Sometimes the unlucky fall victim to a smugness that stems from having survived icing encounters in the past.
Sometimes, however, ice strategy is little more than wishful thinking. Consider the case of a pilot of a Part 135 Aero Commander 500B, who was concerned about the icing conditions for his flight to Hays, Kan. He was concerned enough to get two weather briefings. Neither was very encouraging. Both briefings advised of a Sigmet for moderate to severe icing in freezing rain below 4,500 feet.
The pilot then called his dispatcher and chief pilot, who advised him that they thought the weather had not actually developed as it was forecast. The weather that concerned him appeared to be east of Hays, his first landing location, and the pilot decided to make the flight.
He arrived at Hays to find the weather as had been forecast. The descent into Hays loaded the airplane up with ice. The pilot missed the field at MDA and began to execute a missed approach. Maximum takeoff power was applied, but the Aero Commander wouldnt climb and settled into the ground with full power going. Fortunately, only one injury resulted.
The aircraft was not equipped for flight into known icing conditions, although it did incorporate an STC for the installation of rubber strips on the leading edges. The strips are sprayed with glycol to prevent ice buildup. The pilot stated that the owner had represented the STC equipment as meeting FAR 135.227, for flight into known icing conditions, even though it did not.
The Icing Season
Airframe ice claims an estimated 40 to 50 airplanes each year. The evidence is often melted by the time investigators get to the scene of a crash, so they use a tool known as backing the aircraft up out of the hole. What, they might ask, was the condition of the airplane two feet from impact? What was it like at 100 feet from impact, 200 feet, 500 feet and so on. Not all of those answers can be easily answered.
When it comes to icing, experts have to examine whether the conditions that existed could have caused icing sufficient to change the center of gravity or aerodynamics of the wing and tail structures enough to cause the pilot to lose control of the aircraft. Was there enough extra speed and power to cope with degraded aircraft performance because of the ice load being carried?
The number of ice-related accidents provides evidence that general aviation pilots typically are poorly trained at detecting icing conditions before takeoff and coping with them in the air. Flying in icing conditions is often a way to make a bad day worse – in a hurry.
The primary attack ice uses is to blot out the wings ability to create lift by destroying the smooth flow of the air. It adds drag, which calls for a power increase to take care of the added drag. Angle-of-attack may have to be increased to hold altitude. An AOA increase means that the bottoms of the wings and fuselage will also accumulate ice. While ice adds weight, it causes the most trouble by disrupting the airflow around the airplane.
Every exposed part of the aircraft can accumulate ice. Cowlings, intake areas, vents and antennas are critical areas, as well as the wings, propeller and windscreen. An airplane can take on so much ice in a short time that it cant maintain flight. Ice can get into crucial spots where even de-ice fluid and boots cant get rid of it.
Once encrusted, the airplane will stall at lower angles of attack than usual, and at much higher speeds. The ice will create an experimental airfoil out of your otherwise trusty wing and the airplane may pitch and roll in flight to the point where recovery isnt possible.
The Strategic View
The best weapon in dealing with icing conditions is conservative judgment. Making the call begins with getting a complete preflight winter weather briefing. If the icing weather is against you, cancel and wait for conditions to improve. It may mean going back to the hotel for another night, making a few phone calls and settling down to a good dinner. The alternative course of action – gutting it out – may be expedient but can have a decidedly unhappy ending.
Assessing weather conditions for making a go decision depends on a number of factors. The primary factor is that visible moisture and temperatures between 0 and -20 degrees Celsius are what you avoid.
Suppose you (or your passengers) have decided that the flight is possible and necessary. Add these items to your criteria for safe cold weather flight:
• Are the cloud tops low enough to allow a quick climb to on-top conditions? Do the conditions require a turbocharged power plant up front? If so, is your airplane so equipped?
• Will the cloud bases be high enough to allow descent to ice-free altitudes below the clouds? Will those low altitudes have temperatures above freezing?
• Are there good ice-free alternates that are close to the planned route of flight or, at the very least, alternates with conditions far above marginal VFR?
• Is the flight planned along a route that will be above relatively flat terrain?
But perhaps the biggest stumbling block to assessing the threat from icing is that ice is unpredictable. The forecasters do the best they can with the data and weather models they have to work with.
They do err on the side of caution, however, so if conditions are reasonable for ice youll hear the forecast. That doesnt mean, however, that absent a forecast for ice youre in the clear. Sometimes the briefers dont mention icing and the first clouds you encounter plaster the airplane with a heavy coat of rime.
Dont get in the habit of thinking the briefers are always right or always wrong about ice, just as you know theyre not always right about cloud levels, winds aloft, ceilings or thunderstorms. Use their information to arm yourself with outs should the weather take a turn for the worse.
Ice on the Ground
Ice can strike an aircraft any time there is precipitation or moisture in the air, combined with below-freezing temperatures. Even a thin layer of frost can severely disturb the smooth flow of air over a wing.
The easiest method of keeping the airplane ice and snow free is to park in a hangar. Of course, thats not always possible. Even if the airplane accumulates ice when parked outside, it can usually be deiced by a few hours in a maintenance hangar that has even a modicum of heat.
Once the airframe is clear of ice and snow, wipe it dry if freezing temperatures exist outside. Even that may not be enough, however, as hinges and other control surface components have been known to freeze. The FBO may have de-ice equipment and fluids available as well.
Light accumulations can spell trouble, but pilots seem to subconsciously minimize the potential for trouble. Here is an accident that occurred at Steamboat Springs, Colo., to a Cessna 421B.
Just prior to takeoff, the pilot was observed brushing snow off the wings. Witnesses reported the airplane had ice on the wings and horizontal stabilizer, and icing conditions were present. After loading nine people on board the eight-place aircraft – which put the airplane 258 pounds over maximum gross weight – the airplane attempted a takeoff, but crashed shortly after lifting off. Following the accident, granular ice was found on the aerodynamic surfaces.
Icing in Flight
Many rookie instrument pilots are surprised at how easily structural icing can occur. All you need is some visible moisture and an outside air temperature at or below freezing. Depending on the temperature range, the visible moisture droplets can fall out of clouds as snow, sleet, rain or drizzle. Many times these moisture droplets are in liquid form but have been cooled to temperatures below freezing.
If an airplane strikes these super-cooled droplets – which like to hang out in stratocumulus clouds – the result is an immediate layer of opaque, frosty crust.
The amount of ice that accumulates is in direct proportion to the amount of liquid water that is in the clouds. If you run into large droplets with a high liquid-water content, they can run back over the airfoil, leaving a trail of clear ice. This stuff is really dangerous.
It can build rapidly into some strange horn shapes that totally disrupt airflow over the wing. The ice increases drag and decreases lift. Maximum lift is greatly reduced – and will occur at a much lower angle of attack.
If that isnt enough, the ice on the propeller blades makes the prop similarly less efficient. Once you get to full power and a high angle of attack to maintain altitude, you can expect ice to form on the underside of the wings, tail and fuselage.
A small ice buildup can decrease lift by 30 percent and increase stall speed by 15 percent. The ice that wraps smoothly around the wing can increase drag around 200 percent. If you are unlucky enough to get the classic horn shape of clear, or mixed icing, then figure drag increases on the order of 300 percent to 500 percent. Spot the need for immediate action?
Temperatures that range from 0 to -10 degrees Celsius are the most likely times to find icing. You seldom find icing at -20 degrees C or colder, but its not a hard and fast rule – it can happen.
The type of cloud formation also gives hints as to their icing potential. The clouds with the least icing potential are stratiform. Usually the icing layer in stratiform clouds is thin – about a thousand feet thick – but they can cover a great expanse of country. A change in altitude can usually (but not always) get you out of the ice.
The worst icing will be near the top of the stratus cloud layer, where there is the most moisture and where the moisture droplets are largest. Stratus clouds that form over or downwind of large bodies of water can contain severe icing conditions.
If stratus clouds are lifted mechanically by being blown over hills or are lifted by convection, they can become stratocumulus clouds that produce heavy rime icing. It can be the most severe icing of all. The best course of action to save the day will be a change of course or a descent to warmer air.
Snow is seldom a major problem in flight. Because it is already frozen, it wont normally stick to the airplane. However, it will stick when its heavy wet snow – stuff with a high moisture content. Depending on where you are, climb, descend or divert to get away from it pronto.
When moist warm air aloft overruns sub-freezing air that is at the surface, airplanes are facing a serious problem – freezing rain. When rain forms in the warmer air and then falls through the freezing layer, the product is instant clear ice. Plastered on your airplane in seconds is layer after layer of moderate to severe icing.
You say your airplane is certified for flight into known icing conditions? Freezing rain has brought down the best of them – large and small. Avoid any area where there is freezing rain. Play this one conservatively: Reverse course and land at the nearest airport.
The best way to cope with icing is to always have a place to shed the ice. You start with a complete understanding of what you are going to face as you fly into an air mass ahead. You need to have a pretty good idea if you can descend to warmer temperatures or if you can climb to cold and clear on top. Barring either of these, expect to land promptly.
If you decide to go, stack the odds as much in your favor as you can. Check your pitot heat, prop heat/slinger and whatever other de-ice equipment you may have. As you taxi out, steer clear of standing water or slush. If you have airframe contamination now, you might want to reconsider the flight entirely, because odds are it will get worse before it gets better.
Even if you have an airplane equipped for K-ice, dont be in too much of a hurry to get on top of the clouds unless you know where the cloud tops are and are reasonably sure you can get there. Stratocumulus can be a thick ice factory.
If you cant get on top due to an ATC delay or a deteriorating climb rate, let the controller know right now. You may need to get a vector away from traffic to continue your climb to on top or a descent. ATC will help, but they need to know the problem. Youre here, so dont let pride or a fear of enforcement delay getting help. You dont want to wait until the airplane is so loaded down with ice that all it will do is to descend.
Consider the plight of the pilot of a Cessna Caravan who was flying from Tulsa, Okla., to Ardmore on a Part 135 cargo flight. The weather wasnt something that a weekend instrument pilot would want to venture into. The destination was overcast at 700 feet, visibility 1.75 miles in fog, temperature 26 degrees F and dew point 25 degrees F. There was also moderate freezing rain and freezing drizzle in the area.
The pilots experience level was gold-plated. His ratings included commercial and flight instructor for single- and multi-engine land. His total flight experience was 4,374 hours, with 2,065 hours in the Cessna 208B. In the previous 90 days he had flown the 208B 180 hours, with 4 hours in the 24 hours prior to this flight.
The pilot received a full weather briefing from the Flight Service Station by telephone and departed for Ardmore. Things were going quite well – up to the arrival. The pilot began an NDB approach to the airport, and structural ice began to accumulate on the airplane at a rapid rate. While the pilot was executing the procedure turn the ice load increased.
As the airplane began to lose altitude, the pilot applied maximum takeoff power on the turboprop engine, but it wasnt enough to stop the descent. After breaking out into visual conditions, the pilot elected to make a forced landing in an open field two miles short of the airport. He was unable to arrest the descent and the airplane struck the ground in a five-degree nose-down attitude.
The pilot later stated that his forward visibility was completely obscured by the accumulation of clear ice on the windshield. The airport manager had informed the pilot that freezing drizzle was present at the airport when the pilot had called the Unicom during the initial part of the instrument approach.
The NTSB held that the cause of the accident was the pilots continued flight into adverse weather conditions prevailing at the destination airport and the pilots inability to maintain a visual lookout due to windshield icing.
Approach and Landing
Before starting a descent, while you are coasting along on top of heavy gray clouds that you suspect may be full of ice, get ATC to let you stay on top as long as possible. Then you want to descend through the icing conditions as quickly as possible, recognizing that at this stage of the game theres really no such thing as a little ice.
If you do pick up ice, dont disturb the airflow over the tail airfoil by lowering flaps on approach. It can cause the tail to stall, which will be followed by a rapid pitch down. Recovery at low altitudes from this abrupt stall is not likely.
If the freezing level is near or at the surface, there is a very good chance youll still be carrying any ice you pick up in the descent when you arrive at your destination. You may not be able to see through your windshield, although the defroster may clear a little spot. If youre desperate, some pilots report success with the time-honored method of reaching out the window and scraping a spot with a credit card.
If you cant get enough heat out of the defroster – and you usually cant in general aviation airplanes – turn off the cabin heat, which should give you more defrost potential. Sometimes it helps to fashion a funnel out of charts and notebooks on top of the glareshield to retain heat on the windscreen. Remove before landing, as they tend to fly around the cockpit at the wrong time.
If that doesnt work, youll need to slip the airplane as you cross the fence to line up the airplane through the side window. Then, just before touchdown, kick out the slip with rudder and land using peripheral vision.
Its easy to lose control of an iced-up airplane during the approach or landing. The stall speed and the handling characteristics may differ considerably from those of a clean airplane.
You need to be very cautious when making turns while maneuvering because the potential for a stall is high. The thicker the ice, the higher the approach speed should be. A 20 percent increase in the final approach speed is certainly called for, even with a thin coating of ice. Still, once the landing gear is down and locked, make the power adjustments small and smoothly. The extra speed will increase the stopping distance after landing.
Use the longest runway available. There can be ice or snow on the runway. You may need all the runway length you can get.
Flying light aircraft during icing season isnt impossible, but it is more difficult and takes a greater understanding of the weather and the airplane. The strategy for success is short and sweet: Plan ahead.
-by Ray Leis
Ray Leis is an ATP, CFII and FAA Aviation Safety Counselor.