About the Author

My photo
Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Sunday, December 5, 2010

The Top 10 Considerations for Winter Operations

Well, winter is certainly here. At least it's here on the prairies, and here to stay. We had a -18C day today. I'm new to the prairies, but I'm told it gets quite a bit worse... That would be ok if the wind would just stop filling my driveway with snowdrifts!

So, since winter is here, it's time to have a good look at winter ops. A quick and easy way to do that is with a top 10 list. So, without further ado, the top 10 considerations for Winter Operations (in no particular order):
Engine (and Battery!) Pre-Heat

Engines can be finicky when they're cold. The two main problems are the oil viscosity (although this problem is reduced with modern, multi-viscosity oils) and the reduced vaporization of the fuel—liquid fuel doesn't burn, it must vaporize and mix with air first. These problems are aggravated by reduced battery performance in cold conditions.

To improve the start-up reliability of an engine in cold weather, pre-heat the engine before starting. The easiest way to pre-heat is in a warm hangar, but an engine heater can also be used. An engine heater is simply a supply of warm air (warmed electrically or by combustion) pumped into the engine cowling. Heater effectiveness can be improved by using a cowl cover—which is essentially a form-fitted blanket that wraps around the engine cowl to help trap heat. Having a cowl cover provides the extra advantage that during short turnarounds, the cowl cover can help the engine hold the heat from the previous flight, thus eliminating the need for an engine heater.

Engine Warm-Up on Start

After the engine is running, it still needs time to come up to full operating temperatures. As long as there are strong temperature gradients in the engine, there will be unnecessary mechanical stresses. Running the engine at high power settings prematurely can lead to damage and a need for extra maintenance and/or early overhaul. Hopefully, you have a cylinder head temperature gauge, as this is the most important temperature indication for winter ops. But if you don't you'll need to use oil temperature as a proxy. This is less accurate and less reliable, but sometimes a necessary evil.

There is some good information on engine pre-heat, start-up, and warm-up HERE.

Shock Cooling

Shock cooling of piston engines during prolonged descents is always a concern. But it becomes much more of a concern during winter ops thanks to the increased temperature difference between the engine and the cooling air. Having a winterization kit helps (by reducing the cooling airflow) if that's available, as does closing the cowl flaps if you have them. But in any case, you should carry partial-power during all descents. If that won't get you down fast enough, use flaps, gear, spoilers, or a finer propeller setting—or even better, fine-tune your descent planning and start down sooner.

It's also a good idea to warm the engine by spending a few seconds at cruise power (or full power for lower-powered engines such as those on most trainers) every couple of minutes. This can be referenced to altitude instead of time (for example, every 500 or 1,000 feet) for the sake of keeping track.

Warming the engine has the added benefit of confirming that the engine is still actually running. At low power settings in a descent, an engine failure may be difficult to recognize. Further, as the engine cools, the quality of combustion in the cylinders can be reduced—sometimes resulting in an outright engine failure. Identifying a failure sooner means you have more time (and altitude!) to deal with it.

Note that there's a pretty good argument against the very existence of shock cooling HERE, but until more data is in regarding the problem, I'd stick with conventional wisdom. Worst-case-scenario then is that you're erring on the side of caution—not a bad course of action in an airplane!

Critical Surface Contamination

Surface contamination can occur year-round in flight under the right conditions. But only during winter operations will you see frost and icing on the ground. Inspecting for and eliminating this contamination is a vital part of your winter pre-flight inspection.

Surface contamination has two key and detrimental effects—it increases drag and decreases lift. The ultimate effect of both of these is to degrade performance—sometimes to the point where retaining control or remaining airborne become impossible. Surface contamination will result in reduced cruise speed, reduced climb rates, reduced glide range, increased stall speed, increased takeoff distance, and the potential for control malfunctions—including things like aileron reversal and tail stall

Dealing with in-flight icing is a topic unto itself, and is briefly touched upon below. But dealing with ground icing is easy—get rid of it. Any adhering ice or snow (including frost) should be removed from your aircraft before departure. You can do this during pre-flight with brushes and scrapers or in a warm hangar, or you can get your aircraft de-iced with de-ice fluids before takeoff. The choice between these two approaches will depend on the available facilities and your available budget. Either way, plan ahead and make sure your schedule accounts for the time required and that you have the required equipment.

Further information regarding ground icing can be found in NASA's excellent online course right here.

Runway Contamination

Contaminants on the runway (water, slush, snow, ice) can hit you with a good one-two punch. They degrade acceleration (on take-off) and deceleration (on landing) plus, they degrade directional control. This can result in performance degradations leading to runway over-runs, or loss of control resulting in going off the edge of the runway. In extreme cases of landing on thick snow, the result can even be flipping over and stopping very short after the landing gear digs in.

Avoiding runway contamination completely isn't always an option. So the key is to be aware of contamination present and to be familiar with your aircraft's limitations (and your own!). When considering these limitations, don't forget to account for the crosswind component and the gustiness of the winds.

At airports with ground facilities, such as control towers or flight service stations, advanced notice of surface contamination is often available in the form of Runway Surface Condition (RSC) reports. At uncontrolled aerodromes with no ground stations available, you may be able to get reports from other pilots who have taken off or landed there recently. In the absence of these reports (or if you doubt their reliability), a precautionary inspection of the surface is called for. If you're taking off, this can be done by taxiing (or in extreme cases, walking) the length of the runway. If you're landing, conducting a low-level inspection would be prudent—as would including fuel for a possible diversion in your flight planning

Airborne Icing

Airborne icing can occur year-round, so it really isn't a winter-specific problem. However, it is more common during the winter. It's also far more likely in the winter to occur outside of cloud—for example, in precipitation or mist—making it a much more significant concern for VFR pilots during winter than during summer.

Unless you are in an aircraft equipped for icing conditions and you have received training in operating that equipment and dealing with in-flight ice, the best way to deal with the ice is to avoid it. This means checking forecasts and reports thoroughly prior to departure. It means getting weather updates while en-route. And it means developing an in-depth understanding of the weather and the conditions that may lead to icing.

If you find yourself in icing conditions (without the equipment or training to deal with it), deal with the problem like any other emergency—and yes, this is an emergency—keep flying the plane, and assess before acting rashly. It's often a good idea to undo the last thing you did. If you were in cruise when you entered ice, turn around. If you were climbing, descend. If you were descending, climb (if you still can).

In any case, your objective needs to be met in two steps: first, stop any further ice accumulation; second, get rid of the ice you have. The first step means getting away from liquid water. The second means finding some warmer and drier air to get the ice to sublimate off. Both of these objectives can often (but not always) be met by changing altitudes. As you're dealing with the problem, talk to ATS and/or other pilots to get more information on where the icing conditions are. Also, pass the word on so other pilots can avoid the conditions you've found yourself in.

Further information regarding in-flight icing can be found in NASA's excellent online course right here.

Cold Weather Survival

If you're lucky, you get to do your pre-flight inspection in a warm hangar, and you have an airplane with a good heater. That makes winter flying much more comfortable and enjoyable. However, it can lull you into a false sense of security. If your heater fails (or if you have to shut it off due to CO poisoning, discussed below), or worse, if your engine fails and you have to spend a night in the woods, you'll be very sad if you only dressed for a warm hangar and a good heater. If you're lucky this time, you'll only lose your fingers and toes to frostbite. If you're not so lucky, Search and Rescue will become Search and Recovery. Even barring an emergency, un-forecast weather can force you to make an unplanned landing off-field or on a small middle-of-nowhere aerodrome with no facilities.

Keeping all of this in mind, you should always be dressed for the walk home. Sure, you can (and should) store some extra stuff (including blankets and fire starter) in the back. But imagine crashing and being pinned in your seat unable to reach that stuff in the back—or narrowly escaping from burning wreckage only to watch your blankets go up in smoke. Won't you be glad you turned the heat down and wore some extra clothing while flying?

Cold weather survival is something that deserves attention during winter ops. I recently heard a statistic from some military folks: Average Search and Rescue time is 48 hours. So if you're on the ground and nowhere near civilization, you need a way to keep warm.

Spatial Disorientation due to Whiteout

Whiteout occurs when an overcast sky visually merges with a featureless, snow-covered ground—eliminating the horizon from a pilot's visual reference. This condition can lead to spatial disorientation and a subsequent loss of control, even in clear weather with good visibility. The problem can be compounded by empty field myopia, which sets in in the absence of features to focus on. Whiteout can also be aggravated by reductions in visibility, especially by falling snow, even when light.

Increased instrument reference is a must in whiteout conditions. If you find yourself disoriented, try to change directions (with instrument reference) to find ground features that will help you regain orientation. Hills, mountains, heavily forested areas, bodies of water that haven't frozen yet, large built-up areas, etc. can help break up the snow cover of the ground, and thus reduce or eliminate the whiteout.

Carbon Monoxide Poisoning

In the winter, we tend to seal up the cabin as best we can, and fly with the heat on. There's a good reason for this: It's COLD! The downside to this quest for warmth is that it increases the risk of CO poisoning. Heat is supplied either by a heat shroud over the exhaust manifold/pipe, or by a combustion heater. Either way, there's a risk of combustion products leaking into the cabin.

The best way to deal with this risk is to be equipped with a CO detector. The detector you use should be sensitive enough that it will sound the alarm before any negative physiological effects kick in. And what do you do when (if) the alarm goes off? 1) Turn off the heat, 2) Open the vents and windows, 3) Land. Yes, it's going to get cold. So dress for the walk home.

The Pre-Flight Inspection

Although we've already looked at the engine and battery pre-heat, as well as critical surface contamination—both of which are key elements of the pre-flight inspection—it's worth revisiting the pre-flight from a broader, human factors, perspective. It's cold out! Human nature being what it is, cold is a strong motivator to hurry it up and take shortcuts. It also creates a significant distraction—we focus on staying warm instead of inspecting the aircraft. In a word, this is dangerous.

There are two key steps to addressing this problem. The first is to be aware that there is in fact a problem and to make a conscious effort to focus on the task at hand (i.e. – the pre-flight inspection). The second step is to dress warm. If you're dressed for the weather, the weather will be less of an impediment for you. This goes back to cold weather survival, too. If you're carrying winter gear for an emergency, why not use it for normal operations as well?

An even better solution, when the option is available, is to complete your pre-flight inspection inside a heated hangar. This keeps you warm—helping ensure a quality inspection—and it keeps the engine warm until it has to go outside.

No comments:

Post a Comment