December 2001 Volume 44 / Number 12
Bruce Landsberg has served as executive director of the AOPA Air Safety Foundation since 1992.
Snow falling softly through the trees and onto the fields is one of nature's prettiest sights, but it's something that should make pilots uneasy. Contamination is the professional's term for snow when it builds up on runways or the wing and tail surfaces of our aircraft.
Patience is usually a virtue in flying. When pilots hurry, mistakes are made. But there is at least one exception—the faster one gets airborne the better when threatening weather is moving in. Last January a Piper Malibu Mirage was substantially damaged during an aborted takeoff. In the left seat was the owner, a private and instrument-rated pilot. In the right seat was a CFI with an airline transport pilot certificate. A business associate was the only passenger.
The Piper was used for business travel between job sites, and the pilot was cautious about flying. The pilot's flight instructor accompanied him when the weather was forecasted to be "hard IFR." With only 672 total flight hours, including 133 hours in make and model and 72 hours of actual instrument time, it was prudent to have some more experienced help in this high-performance aircraft.
The flight had landed mid-morning for the pilot and passenger to attend business meetings in the local area. They returned to the airport between 1 and 2 p.m., when heavy snow began to fall. The airplane was kept in a heated hangar during the morning hours—a good move when snow is forecast. At departure time it was towed out of the hangar and fueled. The occupants climbed aboard, and the Piper was taxied to the end of the runway. Up to this point the trip had been a model of good winter flight operations; however, things began to unravel slowly.
Reaching the run-up pad, the pilot performed the before- takeoff checklist and then both pilots used their cell phones to call flight service. The CFI received a weather briefing, filed an IFR flight plan, and requested an IFR clearance. Since the flight service station reached was not primary for the airport, the briefer placed the caller on hold to request the IFR clearance from center. The call was disconnected and the instructor called the FSS back.
As many of us have experienced during inclement weather, there was an extended delay to reach the briefer. Other than the loss of time and on-hold music better suited to other people's tastes, there usually is not an operational imperative to get moving—except when heavy snow is falling. The CFI finally called center directly, and received an IFR clearance and void time. The whole process took about 45 minutes. The snow continued to fall.
The private pilot performed another engine runup just to be sure that the plugs hadn't fouled because of the prolonged idling. He also noted that about three to four inches of slush were on the runway. The pilot's operating handbook (POH) on most light aircraft does not discuss the effect of slush or snow on takeoff performance but in FAR Part 25, "Transport category airplanes," anything over half an inch is considered a big deal. One large-aircraft flight manual (for the Lockheed P-3) states, "During takeoff, treat everything as wet (dense) slush or standing water. With runway conditions having more than three-quarters of an inch of slush or standing water or dry snow in excess of 5 inches, takeoffs are not recommended." An accompanying chart shows that three-quarters of an inch of slush will increase takeoff distance by nearly 30 percent, and this is with an aircraft with a much higher power-to-weight ratio than most light airplanes. The manual goes on to say, "Be aware that the retarding force of snow, slush, or water acting on the wheels is proportional to the square of the forward speed: hence it is most pronounced in the later stages of the takeoff run." That subtle statement means that you may not know how bad it's going to be until you're moving down the runway at high speed. So it could be too late before you know if there is enough energy to fly or enough room to reject the takeoff.
If given the option, go early in a snow event before there is much accumulation and find out if the airport has snow removal equipment to clear the runway. If they don't have dedicated plows or blowers, it's a good bet that the state highway administration will get around to it in its own sweet time, which probably won't be as soon as you would like. Airplanes are not good plows and there is a limit to what they can handle. Unfortunately, it usually isn't published, and for FAR Part 23 or CAR 3 aircraft, the manufacturer doesn't know either.
If runway slush wasn't enough to prompt reconsideration by the Mirage pilots, they also noted about one-half to three-quarter inches of slush on the upper surface of both wings. They discussed returning to the ramp to wipe off the wings. Both agreed that the slush would "blow off" during the takeoff roll and that it was not an issue. Returning to the ramp would have meant missing the void time, a lengthy delay while the aircraft was cleaned up, time for still more snow to accumulate on the runway, and another round with FSS or air traffic control to get a new clearance and void time. Better to take the chance and go now, they decided. Three minutes had elapsed since they had received the IFR clearance.
During the takeoff roll, the pilot stated that it took a long time to get to the rotation speed of 80 knots. He estimated that he used 4,500 feet of the 5,400-foot-long runway to reach 80 knots. When he pulled back on the control column, the airplane did not lift normally, and he stated to the instructor that "it did not feel right." He asked the CFI to feel the controls; he confirmed that "it did not feel right." This was probably not the best time to discuss an aborted takeoff, since the aircraft was covering about 117 feet per second and options decrease rapidly at that speed. A rejected takeoff decision should be considered in advance, and the pilot should have a distance or location in mind that represents the no-go point. If it's not flying by then, it's time to stop.
The Piper was now about three to four feet off the ground, barely flying, and rapidly running out of runway. The pilot turned right to avoid trees off the end of the runway and retarded the throttle. The airplane touched down, slid, turned, and came to rest upright facing the runway. All three occupants exited the airplane with no injury. Rejecting the takeoff probably saved them from serious injury or worse.
The pilot estimated that about an hour had elapsed since the airplane was pulled from the hangar until the time of the accident. An FAA inspector arrived shortly afterward to examine the airplane. The main landing gear was still attached to the airframe, but was bent to the right. The nose gear was intact, and there was no damage to the engine or propeller. Both wing tips were bent, and the left aileron, flap, and wing exhibited impact damage. The fuselage was wrinkled along the area behind both wings. No other mechanical deficiencies were noted.
The inspector reported that beneath the snow that covered the wings were localized patches of ice near the trailing edges. The aircraft POH stated, as they all do, "Do not attempt flight with frost, ice, or snow adhering to the exterior surfaces of the aircraft or landing gear."
When snow is falling, some planning is essential to minimize the ground delay. The airlines have learned this lesson the hard way several times (see "Safety Pilot Landmark Accidents: Cold Realities," January 1999 Pilot) and have developed procedures to resolve the issue. First, we will assume that the aircraft is approved for flight in icing conditions. That eliminates much of the GA fleet but some aircraft, such as the Mirage, are approved. This reduces the hazard of icing conditions that may be encountered aloft.
Fuel the aircraft on arrival and keep it in the hangar until departure time. Get the weather briefing and the IFR clearance while the airplane is still covered. Then load the passengers and program the nav equipment before engine start. Now, just like the airlines, with everybody in their seats and ready to go, have the aircraft towed out, start the engine, and make for the runway without delay. A minute or two on the run-up pad should be sufficient to perform checklist items that can't be done before starting. Don't miss anything, but don't dawdle either.
If hangar space is unavailable, then it's time for a brush-off or a spray to remove the snow. With cold temperatures and no melting, a soft broom or brush will do the job. My luck has generally been that there's plenty of water in the snow I've encountered, which means it freezes on the wings and tail. After brushing, the aircraft has to be sprayed down with deicing fluid. The approved fluid is reasonably friendly to the environment and, depending on the composition, may adhere to the surfaces to provide some residual benefit. Non-icing azproved aircraft should not use this as the poor person's solution to getting through the cloud deck if ice is reported.
A snowy departure must be carefully choreographed. Delay the hangar exit or deicing and preheat until the last possible minute. Flight plans, clearances, and loading should be done so that the time spent on the ground unprotected is minimized. The airlines typically specify a maximum amount of time that they can sit before they have to go back to the deicing pad. If you have a major ATC delay it may be necessary to return for additional deicing.
With apologies to Robert Frost, pausing by the woods on a snowy evening is best not done in aircraft.
See also the index of "Safety Pilot" articles, organized by subject.