Twenty below outside, and the cockpit air conditioning set to full cold. It sounds like a mistake, but for A-10 Thunderbolt II pilots flying in Alaska in winter it was common practice. Former US Air Force A-10 pilot Lynn Taylor explained the reason in a Quora answer that The Aviation Geek Club published this week, and it has less to do with the aircraft than with what the pilot was wearing1.

Dressed for the ejection, not for the cockpit

"We would often run the air conditioning at full cold when flying in Alaska in the wintertime," Taylor wrote. The reason was a rule every pilot over cold terrain lives by. "Because you always need to 'dress to egress.' Just in case we had to eject, and had to wait more than five minutes to get picked up on the Alaska tundra in the middle of winter, we wore three layers of very warm clothes." The result inside a sunlit canopy was predictable: "So, it's twenty below outside the cockpit, but you're sweating like you're in a sauna." His conclusion: "Hence, run the ice maker full 'ON.'"1

Taylor described the sun as the main source of heat in the cockpit, "much like the inside of your car in the summer", along with the bulk of the clothing itself1. Sweat matters here for a reason that goes beyond comfort. US public health guidance notes that hypothermia can occur even at cool temperatures when a person is wet from rain, sweat or cold water and becomes chilled, and it recommends several layers of loose-fitting clothing outdoors2. Sweating into the very layers meant to keep you alive after an ejection is therefore more than a comfort issue.

The operational limits were set by the same risk. "We didn't fly when it got colder than -20 for sure. It might have been 0, but I don't remember," Taylor wrote. Beyond a certain point, he added, "if you have to eject, your life expectancy is ridiculously short. You always have to plan for that possibility." At -40 or colder, he recalls, they did not even go in to work1.

Why the A-10 made ice in its own ducts

Taylor admits he no longer remembers "all of the technical wizardry" of the system, but he does remember one side effect: the air conditioning "also doubled as a projectile ice maker". "If it ran on full cold long enough, ice would build up in the ducts," he wrote, and when manoeuvring or taxiing the ice would sometimes break loose and shoot out of the vents1.

The physics behind that is standard textbook material. A fighter's environmental control system (ECS) pressurises the cockpit, regulates its temperature, provides breathing air and removes excess humidity; the same system can also supply avionics cooling, defog, anti-ice and anti-G air3. In the common air cycle design, hot engine bleed air is first cooled in a ram air heat exchanger, compressed in an air cycle machine, cooled again and then expanded across a turbine, which brings it down to near freezing4.

That cold air can no longer hold the water it carried when it was warm, so a water separator coalesces the mist into droplets and drains them away. Because the turbine outlet air can get cold enough to freeze the water in the separator and block the airflow, a refrigeration bypass valve, also known as a 35 degree valve or anti-ice valve, mixes in warm air to keep it above freezing4. Moisture, near-freezing air and long running at the coldest setting help explain how ice could end up in the ducts and, occasionally, coming out of Taylor's vents.

Defog: a separate control and a checklist item

Fogging of the transparency was a separate issue. "There is a separate controller for defogging the front windscreen," Taylor wrote. "I think it is all part of the same air conditioning system, but it is possible to run one without the other." The windscreen defog, he remembers, is one of the steps in the pre-descent checklist, and he remembers it because he once forgot it: "Yeah, that was embarrassing to have to abort a landing and go into a holding pattern while my windscreen cleared up. Shame can be a great teacher."1

The meteorology is the same as on any windshield. The dewpoint is the temperature to which air must be cooled for its water vapour to condense, and below 0 °C it is sometimes called the frost point5. Air reaches saturation when, among other things, warm air moves over a cold surface6. A transparency that has been cold for the whole flight, meeting moist air near the ground, is exactly such a surface. Taylor does not describe the mechanism in his answer, but his checklist step is the practical answer to it: warm the glass before you need to see through it.

What this means in a civil cockpit

Most pilots reading this will never wear three layers of survival gear in an attack jet, but the same systems sit in every transport aircraft. The air cycle system that cools bleed air is what airline crews call the pack, and the FAA uses the Boeing 737 as its worked example of how it operates4. Understanding how the pack, the water separator and the temperature control interact is part of the airframe and systems subject in ATPL theory.

Windshields are handled in several ways. Transport category windshields are laminated and carry electric heating elements that keep them clear of ice, frost and fog, often with a NORMAL and a HIGH setting. Some older laminated windshields route bleed air or conditioned ECS air between the plies to keep them warm and fog free, while small aircraft blow ducted warm air over the inner surface, much like a car. Those warm air systems are not approved for flight into known icing4. In piston aircraft that warm air usually comes from a shroud around the exhaust muffler6, or in some twins from a combustion heater that also serves for windshield defogging7.

For light aircraft without windshield anti-ice or deice, the FAA's icing guidance gives a simple instruction: turn the defroster on high and leave it on, which may help to prevent ice forming on the windshield in flight8. It is the civil equivalent of Taylor's pre-descent step, done earlier and kept running.

Cold soak and the cold weather checks

The cold does not stop at the cockpit. Frost disrupts the airflow over the wing, reduces lift and increases drag, and an aircraft must be thoroughly cleaned and free of frost before flight6. The FAA also warns about cold soak: after a long flight at high altitude, cold fuel in the wing tanks can cause clear ice to form on the upper wing surfaces on the ground, even when the outside air temperature is above freezing and nobody would call it icing conditions8.

Engines behave differently too. In cold weather a turboprop can exceed its torque limit while its temperature limit is still within range, so a pilot who sets power by temperature alone can overstress the engine; an over-torque that lasts more than a few seconds can destroy internal engine components7. These are the kind of points that appear in meteorology and operational procedures questions, and in the cold weather sections of aircraft operating manuals.

The human side

Taylor's story is ultimately about planning for the worst moment of a flight before it starts. Dressing for survival on the ground and then managing the heat that comes with it in the air is a trade-off that every crew operating over cold, remote terrain has to think through, and it is a good example of the thinking behind the human performance subject. The A-10s he flew have since left Alaska: the last operational A-10 sortie from Eielson Air Force Base was flown on 31 July 2007, and the last three aircraft departed on 15 August that year9.

For pilots considering a military route before an airline career, our overview of how military pilots move to the major US airlines and the video on Air Force pilot training at Laughlin show where that path starts.