From the Fight Deck: Baby, It’s Cold Outside! – Aircraft Maintenance

From the Fight Deck: Baby, It’s Cold Outside! – Aircraft Maintenance




by Kevin R. Dingman

It’s winter in this hemisphere, which means it’s colder – colder than when it’s not winter. And although winter is a fine time to add a nip of something to your cocoa and wrap yourself in an afghan near a crackling fireplace, it’s not such a fine time for your airplane out on the ramp.

Cold1This article is about the issues our flying machines aircraft maintenance face during winter ground operations. It’s about being nice to your battery (ies), motor(s), furnishings and equipment when it’s cold outside by using the proper gasses, liquids, pre-heating, and warm-up techniques. In other words: how to convince your airplane to go flying when it doesn’t want to go.

And before you turbine-powered pilots peel off to peep at Dave’s article on the posterior page of this publication, postulating that the preponderance of pearls in this pilot’s column are probably pointed at piston-powerplant-powered-planes, pa-fa and au contraire, mon capitaine. There will be plenty of potentially pertinent pearls presented to you as well, so please be polite and persevere.

Half-Naked

We all become acclimated to the temperature of our environment, and therefore have different perceptions of what temperature feels cold. The Mayo Clinic in RST is a common destination for Wings of Mercy trips, and at my Part-121 job we fly to MSP quite often. The hardy souls in these two locales strip-down if the Hg climbs to 30, and are in shorts by 40 degrees. Bless their half-naked, thick-skinned hearts. In some parts of the country, 50 degrees Fahrenheit is cold.

If it’s 50 in February where I live, we don short sleeve shirts, cartwheel out to the deck and fire up the Weber. The only way to know the temperatures at which our aerospace vehicles feel the cold is to follow the manufacturer’s recommendations, learn from our experiences and listen to the tales of old-timers. Here, in the warm surrounds of your home, office, FBO or cockpit, we will explore a splattering of each.

In the Duke community, 50 degrees and lower is when we pre-heat our 380-hp Lycomings. I’ve often received raised eyebrows or a shaking head when another pilot sees the torpedo heaters blowing up through the cowl flaps into the motors when the temperature drops below 50. A waste of time, they say – it’s unnecessary. Those MSP folks would think it’s toasty at 50 and pilots from TPA would say it’s chilly. We need to select a temperature to pre-heat motors, interiors and avionics. Fifty degrees seems reasonable.

I Don’t Think So

A few years ago I tried to get away with ignoring the recommendations from Lycoming and the Duke Flyers Association regarding cold weather starting procedures. The Duke had been parked outside at the DEN Centennial airport overnight where the OAT had been around 20 degrees Fahrenheit; granted, this is a smidgeon below 50. When the left starter was engaged (the engine closest to the battery), it responded with an undeniable “I don’t think so”. While the Duke enjoyed two hours in a heated hanger, the PIC enjoyed some light reading in the POH about GPU-assisted starting procedures. The engines started just fine after that, but this trip convinced me to pick a number well above freezing to begin preheat procedures. This was also the trip in which I had a very memorable GA experience.

We all know that “to con” or “conning”, means to make contrails; non-pilots call them vapor trails. Contrails can be produced during high humidity if an airplane is in a configuration/attitude of high AOA, such as during landing. More often, we associate contrails with an aircraft at altitude. The white trails are generated by the moisture/temperature reaction of engine exhaust to the cold air. On the flight home from DEN that chilly morning, the OAT at FL 220 was minus 60 Centigrade. The Lycomings in the Duke were making contrails – cons from piston engines! It was reminiscent of WWII bombers and was an inspiring sight; after the two seconds it took to realize that they were not on fire, that is.

In order to convince our airplanes to go flying when it’s cold on the ground, there are things to do before the weather turns cold, and after we arrive at a cold hangar, and both during and after the starting sequence. Installing oil cooler baffles, filling tires and struts with nitrogen, use of multi-grade or low-viscosity lubricants, setting the hangar temperature and removing water from the airframe: all of these fall under the before-it-gets-cold tasks. Use of pre-oilers, pre-heating the engines, and warming the cargo, cabin and avionics fall into the at-the-hangar section. And, finally, monitoring CHT’s, oil pressures, temperatures and delta-P indications, and exercising hydraulic components and propellers, would be in the starting and after-start procedures.

ColdIIYour Jet

For the turbine folks that have politely persevered to this point: paraffin, Prist, anti-ice fluids and fuel heaters are cold weather topics associated with your jet. Frozen water in jet fuel can and does clog fuel lines and filters (i.e. British Air flight 38). Like adding a nip of something to your cocoa, Prist, or any FSII (Fuel System Ice Inhibitor) is added to jet fuel in order to achieve an effect. In jet fuel, the desired effect is to prevent water in the fuel from freezing. For convenience and safety, most jet fuel is pre-mixed with an FSII before it’s pumped from the truck; manually adding the proper ratio is not normally required.

Delta-P ( ΔP ) is a term that means “different, or difference in pressure” and is normally associated with the pressure measured at the input vs output side of a filter. This can be an oil, air or fuel filter. A caution or warning system alerts us if the delta-p reaches a preset value indicative of a clogged, or clogging, filter. Alternate-air is used to solve an induction-air delta-p, an oil filter bypass valve is for an oil filter ΔP, and a fuel/air heat exchanger is used to manage fuel-filter restrictions associated with ice in jet fuel. The heat exchanger uses warm air on a timer to heat fuel before it passes through the filters. The relatively-warm fuel will then melt ice and any congealed paraffin in the filters (Kerosene is derived from Greek: keros, meaning wax – a component of jet fuel).

It’s recommended that oil coolers be warmed before engine start to prevent extreme oil pressures due to congealed oil in the cooler. Oil cooler baffles are typically metal plates installed to block a portion of the airflow through engine oil coolers. Some manufacturers and mechanics recommend them below a certain temperature in order to increase oil temperatures. The baffles assist in achieving normal oil temperatures, which are necessary to both decrease oil viscosity and to boil off water deposited in the oil during combustion. After flying or an engine run, I remove the oil dipsticks to allow some of the remaining moisture to escape as steam.

Superior Piloting Abilities

We might think that a heated hangar would solve cold weather problems, and that’s mostly true – but we’re back to the “perception of cold” question. How warm do you need to keep the hangar, and for how long before a flight? And a problem actually created by a heated hangar is that snow will melt as it lands on the warm aircraft skin during taxi, and then re-freeze before or during takeoff. Light, dry snow that may have otherwise blown off will “adhere” to the surfaces, just as it does on warm engine cowlings during a quick turn. And that adhere word is one we are not allowed by the FAA. According to the AOPA Air Safety Institute, during the last 10 years there have been at least 25 accidents on takeoff as a result of wing contamination by snow. More importantly than the regs however, is that a contaminated airframe may cause us to need some of our superior piloting abilities – maybe all of them, and we don’t want that.

Often, the best option is to heat a hangar to about 50 degrees for several hours, followed by the application of an anti-ice fluid if required. Polypropylene antifreeze is pink in color and can be found at RV or automotive stores. A two or three-gallon plastic garden sprayer from the home improvement store works well as an applicator. About four gallons covers the Duke. If away from your home supply of deicer, you may have to purchase aviation Type I, II or IV. Once deiced and anti-iced, if you decide to take off during a snow shower, don’t forget to recalculate accelerate/stop distances using the contaminated runway numbers.

Frightful

There are challenges, tradeoffs and costs associated with cold weather ground operations. Once you’ve establish a routine to prepare the airplane, then preheating, applying fluids and referencing an extra performance chart or two simply adds a bit of time and effort to the launch schedule – it is time well spent. Your alternative is to stay home in front of that crackling fireplace with your enhanced cocoa. And when the weather outside is frightful, sometimes that option is indeed delightful.

DingmanKevin Dingman has been flying for 40 years. He’s an ATP typed in the B737 and DC9 with 20,000 hours. A retired Air Force Major, he flew the F-16 then performed as a USAF Civil Air Patrol Liaison Officer. He flies volunteer missions for the Christian organization Wings of Mercy, is employed by a major airline, and owns and operates a Beechcraft Duke. Contact Kevin at Dinger10d@gmail.com.

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