Tuesday, October 21, 2008
The Go/No-Go Decision In Winter
The rules change when the weather turns cold
|It had been a long day. It was January 2003, and I’d departed Reykjavik, Iceland, in a 58 Baron; destination Iqaluit, Nunavit, Canada, with stops in Greenland, where it was clear and cold—in this case, minus-20 degrees C. I’d landed on the gravel runway at Kulusuk in the dark of noon, refueled as quickly as possible to avoid having the engines cool down, and leaped back off across the ice cap for the old U.S. air base at Sondre Strom Fjord, well above the Arctic Circle. The weather remained perfect as I spanned the cap at 14,000 feet in smooth, frigid air. |
The inconveniences of winter begin with the preflight. Differences appear with simply untying and unchocking the airplane, what my old buddy Bob Pyle used to call “the most important part of the preflight.” Ropes are less pliable in winter, sometimes stiff and inflexible, and chains can be cold and miserable to work with. Checking the fuel caps on a high-wing airplane becomes a lot less fun when the steps on the strut are covered with ice and your fingers are too numb to grab the handgrip on the cowling, so some pilots simply skip it.
Similarly, checking the fuel sumps on a low-wing airplane becomes less fun when you must go down on one knee and have that knee come up wet. Hardly anyone will spend the proper time checking all those things under the airplane that present less of a challenge in summer: wheels and tires, flashing beacons or strobes, antennas, exhaust stacks or cowl-flap security. Snow or ice on the windshield is another problem you don’t normally face in summer, and considering the temperamental nature of Plexiglas, scraping is ill-advised. Break all the credit cards you wish chipping ice off the wings or tail, but windshields are off limits for anything abrasive.
The old cliché that the worst abuse you can inflict on an engine is starting it goes double in winter. All those moving parts out front are cold and unlubricated (a good argument for installing a preoiler if there’s one available), and rubbing metal upon metal causes surfaces to wear each other down, reduces tolerances and eventually contributes to failure. Starting procedures vary from engine to engine, but one general rule is that you can’t overprime an engine in extreme cold. You can avoid most of the start problems completely by paying for a comprehensive preheat at temperatures below freezing, an intelligent investment.
The rules for taxiing change slightly in winter, when ramps may be partially snow- or ice-covered. In any significant wind, keep speed slower than normal and do all braking in a straight line. Similarly, make absolutely certain the engine is up to a normal operating temperature before adding power for takeoff.
While it’s true most piston engines perform better in cold weather, extreme cold can make them perform a little too well. Fly around the northern states or Canada in winter, and you may be faced with a density-altitude problem—exactly the opposite of summer. At minus-35 degrees C, for example, flying from a field elevation of 1,000 feet MSL, the density altitude is minus-5,500 feet, the equivalent of flying more than a mile below sea level. That means a typical normally aspirated engine will pull something like 34 inches of manifold pressure, easily enough to start bending parts and breaking things. Bush pilots who fly the far north in winter know to monitor the manifold pressure on takeoff to avoid an overboost.
Virtually everything on the airplane suffers when the OAT takes a dive. Batteries are less efficient, so starters have less cranking power, an excellent argument for a cart start. Tires become less elastic in cold weather and can even develop flat spots in extreme conditions. Lubricants sometimes don’t lubricate, restricting things that should move freely to immobility. Brakes can become glazed and useless in cold, wet conditions. Switches don’t work as well, door locks may freeze, sun visors can embrittle and even landing gear can cycle slower or not at all.