A little ice? 'Tis the season-time to stop thinking thunderstorms and start thinking ice. And while you're thinking, ask yourself-how much ice is too much?
The good news is that most of these accidents are preventable. If you take off with ice or snow on the airplane, you're both braver and less intelligent than many. Airborne, once the ice is there, something probably needs to be done about it. That said, there are a number of pilots out there who have picked up more than a trace of ice and lived to tell about it. How did they get away with it? How much ice is enough, and how much is too much?
ON THE GROUND
One thing that winter pilots know is they can easily become a test pilot for an entirely new and unknown airfoil, one reshaped by ice, snow or frost.
Simply put, it's foolish to take off with any contamination. You have the opportunity to remove the ice/snow/frost before takeff, why not use it? Similarly, once airborne, the contamination rarely will disappear, unless you know you're launching into an inversion and flying into warmer air.
When your airplane is parked outside on a clear, cold night, frost will form on its upper surfaces. It doesn't appear to amount to much, but most reported frost-on-the-wings accidents show that the airplane does get off the ground and into ground effect--about one wing span high--but can't climb any further. A trip into the trees ensues. In these cases, the wing's top surface is simply too rough to develop efficient lift. It doesn't look too different at first glance, but that surface-up close--is a rough mass of jagged ice crystals, and they disrupt the airflow across the wing. Write this down: Any ice, snow or frost must be removed before attempting any takeoff.
How do we accomplish that? We can use the FBO's approved de-icing fluid and equipment or we can break it up and brush it off. We also could move the aircraft into a heated hangar and let it thaw. If we choose the heated hangar, we need to make sure all water is removed from the surfaces. If it isn't frozen on the ground, it can solidify once airborne, after seeping into hidden locations. On the ground, before takeoff, any ice is too much.
The sidebar on page 6 includes some additional tips on pre-flighting and ground operations when airframe icing is likely.
OUT OF THERE
Once airborne, know that the big secret to dealing with in-flight icing is not really a secret. In fact, it's very simple: Always have a way out. This is true even if you are equipped to fly into known icing conditions. To do this, you need a thorough weather briefing, including a firm understanding of the air mass characteristics where you'll be flying. The sidebar at right lists eight questions to ask during your pre-flight briefing-feel free to add your own.
For example, can you descend to warmer temperatures, if you encounter ice en route? Will there be sufficient terrain clearance? If you climb to clear, cold air on top, can you descend and land safely at your destination or alternate? If "no" is the answer to these questions, cancel the flight.
But ice is where you find it--many a pilot has encountered the stuff well away from where it was forecast to be. The most severe in-flight aircraft structural conditions can occur when the temperature is 0 degrees C and colder. Structural icing, of course, is found in conditions of visible moisture.
Are you planning to climb through an icing layer? It's a workable way to handle an icing forecast, if you have Pireps detailing the cloud tops and you are flying a known-ice-approved airplane.
Lacking that equipment, your plan to get on top might not work. If ATC delays your climb and ice begins to accumulate, let the controller know immediately. If he isn't able to give you a higher altitude, ask for a vector away from traffic to climb.
How much ice is too much on departure? If climbing into clear air, we might be willing to accept a 10-percent hit on airspeed and/or climb rate. Anything more than that drastically prolongs our exposure, making the situation worse. Even if you reach your cruising altitude, the ice might not sublimate as quickly as you'd like. That can make your trip a long, miserable one and raise range/endurance issues. Diverting to an ice-free bolt-hole might be the answer. It depends.
At your cruising altitude, you should have more options than you did right after takeoff. A change in altitude can both get you out of an ice-laden cloud or put you in warmer air. But don't wait too long to request a climb since your climb rate will suffer.
Should you climb or descend to get out of the icing conditions? The correct answer has its foundation in several factors. For example, what's underneath you? If you're already at or near the minimum altitude for the terrain, going down is a bad idea. Too, there could be more ice lower. Climbing is usually best, since terrain is not a factor and it leaves you more options. Refer to your pre-flight briefing to learn how high you'll have to climb.
How much ice is too much at cruising altitude? Again, it depends. A trace on the windshield or leading edge probably isn't worth worrying about in a high-performance retractable. Again, our rule of thumb is based on the airplane's performance: A 10-percent hit on cruise speed is too much ice. In an underpowered fixed-gear single, we'd be a lot more careful, and might not have taken off in the first place.
APPROACH AND LANDING
When making the landing approach with an iced-up airplane, you have a number of new problems. The stall speed and general handling characteristics are going to be different, perhaps radically so. Ice has remodeled the wing and control airfoils. You are your own test pilot. This iced-up model hasn't been tested before.
Generally, the thicker the ice on the airfoil surfaces, the higher the approach speed should be. Certainly don't make any abrupt or large changes in pitch, power, bank or flap setting, during the approach or the landing. You'll need to fly the airplane onto the runway. The higher landing speed may greatly increase the distance that it takes to stop. Add a lot more distance "cushion" if the runway is covered with snow or ice.
Landing in marginal surface conditions is pretty much a reversal of the care you took on the takeoff roll. Keep the crosswind aileron correction in; keep straight on the runway; don't get in any hurry to clean up the flaps, cowl flaps or lights until you are off the runway safely. Taxi very carefully to parking.
How much ice is too much? It depends-on the airplane, on the environment, on the pilot and on the mission. A lightly loaded rocket ship might be unfazed by a half-inch or so. A heavy Skyhawk might stagger through the sky after only a trace.
A little ice might not be a problem if you have options and if there's no strong probability of accumulating more. That half-inch of clear is much less of an issue in VMC at 15,000 feet in your Baron than it is when your Warrior is being vectored for an ILS through an icing layer followed by a likely miss. But now you know enough about where the icing is and how to handle the airplane that picking up a little ice isn't going to be a crisis. Right?
Ray Leis holds ATP, CFII, Commercial and glider certificates and, in an earlier life, was a Designated Pilot Examiner for all ratings.
Eight Questions For Your Briefer
Throughout most of the U.S. from November through March, you can count on airframe icing to be a part of your pre-flight weather briefing. This brings us to some important questions for you to ask your weather briefer--or for you to use your weather briefing tools to find answers--before taking off.
1. WHERE ARE THE FRONTS?
Most ice is in the low pressure centers and in the fronts. Will your preferred route of flight take you close to or through them?
2. WHAT DIRECTION ARE THE FRONTS MOVING?
Where will the fronts be located when you take off and where will they be when you reach your destination?
3. WHAT ARE THE CLOUD BASES?
If you can stay out of freezing rain, and below the clouds, there is little chance of structural icing. But terrain features may make this impossible, or at least risky.
4. HOW HIGH ARE THE CLOUD TOPS?
Most light, non-turbocharged aircraft don't climb well with any amount of ice on them and dramatically lose performance above 8000 feet. Even if you do get on top, can you stay in the clear?
5. IS THERE A BETTER ROUTE?
Flying over country with a low MEA is a good safe idea. The weather is usually much better, with less ice than if you decide to go over mountains.
6. ARE THERE PIREPS AVAILABLE?
These reports deserve a lot of attention at flight planning time (and while en route). Get all that you can to form the best opinion. One Pirep isn't enough. If ice is in the forecast and there aren't any Pireps, that can mean even the pros are staying home. This is the time to review your escape routes.
7. IS IT BETTER TO CLIMB OR DESCEND?
Can you descend to warmer temperatures, if you run into ice en route? Can you climb to cold-and-clear on top, and be able to descend to a safe landing at your destination?
8. WHERE ARE THE ALTERNATES?
If you have to go there, what are the icing conditions between your planned destination and the alternate? Terrain? What about alternates for your alternate?
If you can't get enough clear answers to these questions and use them to develop a solid, ice-free plan, cancel the flight. You may feel that it is absolutely necessary to get to your destination, but what if you never get there?
Pre-Flight And Ground Ops
Airplanes can get iced up just sitting around--they don't have to be airborne. In fact, the FAA is justifiably concerned about pre-flight de-icing inspections and procedures. For example, a recent Airworthiness Directive against Cessna's Model 208 Caravan series calls for pilots to make tactile inspections of the plane's upper surfaces within five minutes of takeoff. Try that at Boston Logan or JFK some wintry Friday evening.
Still, the accident record is replete with stories of airplanes small and large--anyone remember Air Florida Flight 90?--that iced up and didn't get very far from the takeoff runway. Here are some things to think about before launching in wintry conditions:
REMOVING ICE AND SNOW
Frost, ice and snow can be removed by sweeping it off, with a hot water/ glycol spray or just tap water if it's warm enough that the water won't refreeze. We've seen attempts with rubbing alcohol--don't bother. You probably can't get enough of it, plus it's a fire hazard.
De-icing and anti-icing services are available at many airports; use them, even if it costs a few bucks. While you're at it, remember to cover the pitot tube and the static sources to prevent any liquid from being blown into the systems and causing problems. Of course, don't forget to uncover them after the deicing treatment.
Make certain that you check the landing gear for accumulated slush. It needs to be cleaned off, especially on retractables: a frozen extension system can be expensive. Wheel pants on fixed gear aircraft are great slush collectors, too. When the slush freezes at the colder temperatures aloft, you can plan on a locked wheel (or wheels) for landing. Hard on tires, and directional control, if only one wheel is frozen solid. If you plan to operate in snow and slush, the wheel pants should be removed and stored for the winter. When departing a slushy runway in a retract, it's a good idea to cycle the gear at least once to blow off any accumulation and minimize any problems.
NOOKS AND CRANNIES
Ice on the airframe is easy to spot, but there are places where frozen water hides out inside it that can cause some major worries. The fuel system is probably the worst of the hazards. Condensation moisture, from storing the airplane with partially filled tanks, is one. Once you are airborne and the temperature drops, any moisture in a crankcase breather line will freeze. In many cases, this blocks the breather, pressurizing the crankcase. Then a seal or gasket lets go, dumping your oil overboard. This is the beginning of a bad day. Pay attention to control surfaces and even cowl flaps, too. Not only can ice or frozen slush interfere with full control movement, but it adds weight and drag, two things of which we never need more.
Landing With Ice
Most icing accidents occur during the approach and landing phases. Ice changes the airfoil's shape and will increase your stall speed. To compensate, carry extra power and speed on final approach. Plan on at least 10 to 20 knots more speed than normal. In addition to the extra drag and reduced lift a load of ice can produce, the stuff's heavy. And it renders your windshield opaque, complicating even a "normal" approach and landing. Here are some precautions:
USE YOUR TOOLS
Few light aircraft have de- and anti-icing tools like boots or hot props with which to handle ice. Still, make sure your pitot heat is working and that it's on. With an injected engine, don't forget where the alternate air handle is and how it works.
USE A LONG RUNWAY
Remember, the surface may be slippery, making it harder to dissipate the speed you carried into the flare. If your home drome is marginal on a good day, go somewhere else and rent a car.
TRY NOT TO USE FLAPS
The airfoil is already deformed; adding flaps can add drag when you really need more power. Another reason to use the longest runway you can.
If you have to execute a missed approach or decide on a go-around, handle the airplane gently, with smooth application of all available power and slow pitch changes. Use shallow banks. With carbureted engines, advance the throttle smoothly, and then remove the carburetor heat.
Pick an approach that allows a straight-in landing. Trying to circle in low-viz conditions in an iced-up airplane is a recipe for a stall-spin accident. Don't even think of flying a full pattern.
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|Date:||Jan 1, 2006|
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