Winter is on an in-bound- heading to most of North America (difficult as it seems to accept while sitting in Wichita with a local ATIS reporting 44°C weather.) When ‘Old Man Winter’ arrives he’s apt to bring his chilly friends who will- ultimately- induce more than a bit of hankering for summer’s swelter again.

Dave Higdon  |  01st September 2011
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Dave Higdon
Dave Higdon

Dave Higdon is a highly respected, NBAA Gold Wing award-winning aviation journalist who has covered all...

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Winter’s Coming:
Incongruous as it sounds- prepare now.

Winter is on an in-bound- heading to most of North America (difficult as it seems to accept while sitting in Wichita with a local ATIS reporting 44°C weather.) When ‘Old Man Winter’ arrives he’s apt to bring his chilly friends who will- ultimately- induce more than a bit of hankering for summer’s swelter again.

There’s nothing like a walk in a biting wind- bare digits grasping ailerons and flaps- rudders and elevators; or gloved fingers fumbling with fuel caps- checking tires and static wicks- lights and radomes…

Indulging in aviation often makes winter particularly tough to tolerate when we feel the cold wind’s blast upon opening the cabin door on a flight from a balmy- shorts-and-shirt-sleeves locale. It makes a body shiver to consider how wonderful that sounds on a sweltering hot day - and how much we’ll curse the cold when it does actually arrive.

Winter makes aircraft operations tougher- and human functionality more challenging. It often starts cold and gets colder- further challenging people and conveyance. Thankfully- technology- experience and science offer us tools to help prepare pilots and planes for winter’s rigors. Pilots and airplanes both need some special care and feeding at that time of year.

So just how cold can it get outside? By standard day temperature charts- it can reach about Minus 50 Fahrenheit at FL300 (30-000msl). Climb up to FL400 in altitude and plunge down to Minus 67 Fahrenheit. Thankfully- the temperature falls relatively little to FL510 at Minus 68 Fahrenheit… but that’s cold comfort. At the altitudes we fly our atmospheric vehicles we face cold air and cold moisture. Each poses its own individual risks to airplanes and crew alike. Acting together- the combination can be deadly.

Use defines a business airplane but certification standards define approved uses- from the basic certification to fly- to approval to fly in instrument conditions or at extreme altitudes.

Flight Into Known Icing (FIKI)- is another level of approval and every business-turbine aircraft flies approved to do so – approved- that is- providing its equipment is maintained and used according to the approval. But as a standard of use- the definition of many a flight department’s operational specifications treat anti-ice protection as an escape tool.

FIKI is not an icing absolution - only a reprieve. FAA and flight-ops standards vary little in their views of FIKI: make it transitory and make it quick. Climbing to clear- cold and usually dry high-altitude air often means transiting ice-producing conditions; likewise for descents. Holding- on the other hand- is highly discouraged.

Anti-ice tools work well to keep clean leading edges and a good amount of the cord distance on wings’ vertical and horizontal surfaces. Engine inlets receive good protection from ice accretion too- but fuselages get no icing protection and it stands to reason that the larger the fuselage area is- the higher the weight added to the airplane.

A square inch of ice weighs a mere 0.53 ounces…insignificant to an airplane weighing thousands of pounds. Check that number per square foot- however - 4.78 pounds - and multiply that with the square footage of fuselage above the floor line- you’ll be dealing with a significant additional weight. One hundred square feet of ice at one-inch think will weigh 478 pounds. If the ice were two inches in thickness- you would be carrying nearing 0.5 ton extra weight.

The ATR-72 regional airliner accident near Roselawn- Indiana (for example) brought the issue of ice to the fore - but unfortunately not to the point of ending pilots’ continued forays into icing of chillingly dangerous levels.

Ice creates problems for aircraft in many ways unrelated to flying into freezing conditions. The two most-prominent issues here are water in fuel and water pooled in airframe cavities. Both hold disastrous potential – and- while this is a story about winter flying- it’s worth reminding yourself that even summertime holds ice-forming conditions at high altitudes.

First let’s look at the fuel issue: Between fuel additives designed to keep water in solution with the Jet A (above-average kerosene) and fuel-tank heaters- water-in-fuel incidents are relatively rare among turbine aircraft. But make no mistake that they happen. Let’s consider a January 2008 short-of-the-runway- barely-controlled touchdown of a British Airways-operated 777 at London Heathrow Airport that was carrying 138 people.

Descending on approach after a 4-400-mile (8-100 kilometer) leg from Beijing- BA Flight 38 developed a sink rate higher than desired after the two Rolls-Royce turbofans failed to deliver on auto- and human-pilot demands for more thrust. The aircraft landed hard about 1-000 feet short of the runway and was destroyed. No fatalities occurred; 48 were injured- one severely.

Similar incidents elsewhere eventually pointed investigators in the direction of ice crystals in the fuel – even though the crew operated the aircraft as proscribed and fuel temperatures remained within limits. Jet fuel begins to thicken as temperatures fall- which can impact the flow rates and pressures of pumps feeding engines or transferring fuel between tanks. But long before jet fuel begins to be impossible to pump- any water in suspension can begin to freeze into crystals – and it was such ice-crystal accretion that clogged the fuel strainers in the tank and starved the turbofans on BA Flight 38.

With temperatures up at the flight levels so extremely cold- airframe makers employ various methods for keeping fuel flowing. Tank heaters- heat exchangers in the lubrication and hydraulic systems can be placed to transfer heat to the fuel.

Operating a business-turbine aircraft with an eye on fuel temperature is critical no matter what the weather - and that’s just one way unknown icing can hurt.

Unseen- unknown icing’s other dirty work occurs when a liquid – water or de-icing fluid are both culprits – seeps into cavities in the airframe and freezes. This can result in jammed controls- out-of-balance control surfaces and reduced effectiveness of flaps. Doubt this at your peril. Let’s cast our minds back to last December when twice in the first two weeks of the month different crews flying two same-model business jets suffered what one of the NTSB incident reports called “rudder binding” – specifically a control-jam while maneuvering for approach to their respective destinations.

Neither flight encountered precipitation en route; both flights occurred in Visual Meteorological Conditions. What happened was that water had seeped into- and accumulated in the tailcone – where it froze. Both flights concluded uneventfully (beyond the control-system binding) but subsequent removal of both jet’s tailcones revealed ice constricting the rudder’s control cables and binding the pulleys.

There’s something else to remember- too: even low-flying non-pressurized aircraft can suffer from unseen icing issues. Water pooled within an airframe becomes a risk to any airplane- anytime that airplane carries such pools above the freezing level. In the winter- of course- that level starts on the ground for much of the United States.

Aside from the jammed rudders of the business jets- we found reports of control-system jamming on a turboprop twin and several piston singles (with ailerons and flap-use impinged or blocked completely). If water gets into the airframe- you face a possible control-surface imbalance with a block of ice in one part of a wing or control surface not matched on the opposite side. If you were unfortunate enough to get a pound or two of ice sitting far enough aft in the fuselage- you could fly out of the CG range – regardless of how carefully you plotted the load- the fuel and the occupants’ weights.

Aileron imbalances may be apparent if – absent the affect of wind – one of the flight controls keeps working to correct a roll-trim problem- or if the aileron continues to drop after you center the control yoke. CG issues- however- may not show up until you’re airborne- wishing that you weren’t.

De-icing systems can’t remove a chunk of water frozen in an aileron or elevator- trim tab or flap- gear leg or truncheon. The solution for suspected internal icing is a warm hangar and several hours without flight. When in doubt- thaw it out - after which- you should cycle all the control surfaces- and assure yourself of a clean- ice-free airplane - inside and out.

Many of the aforementioned issues are wholly avoidable. Keep fuel temperatures above Minus 30 and you’ll be OK; use conditioner to keep water in suspension and you ought to be more than OK. Perhaps the most-difficult aspect of winter flying comes in the pre-flight inspection. Walking around on a bone-chilling ramp is no fun. But the job of Pilot-in-Command and First Officer demands attention to the airframe.

Bare-handed touching and checking of hinges- bolts and safety wires can be chilling; hand warmers in pockets help more than insulated gloves – which can insulate your fingers from noticing something important. And then you arrive at that expensive powerplant. Lubricants inside the engine and its heat exchangers suffer from the same physics traits as Jet A- tending to thicken and flow less as the air temperature falls.

Fortunately- engines and lubricants don’t feel wind chill the way human skin can; but the temperature is still the temperature - and when it gets to below 30- the bearings- seals and pumps all work more happily with a pre-heat.

That can be an overnight period spent sitting in a warmed hangar- or a close encounter with a propane-powered flame and hot air blown into the inlets and heat-exchanger vents. There’s little sense in accelerating that trip to overhaul - a pre-heat can be a cheap investment compared to something failing on start due to the cold.

Winter brings on conditions conducive to skating the airplane along a runway surface. Imagine landing on a runway with reduced braking action due to either ice or snow- or from hydroplaning (a threat that knows no seasonal exclusivity). Ice and snow should be self-informing; if your flight-planning flags up information that the destination runway has degraded braking action due to slush- snow or ice- a call to the airport is the minimum action warranted. Only first-hand information is acceptable.

If the worst is confirmed- the lowest possible touchdown speed is called for with the nose allowed to make a firm arrival for steering. Do not push on the yoke- lest you lighten the weight on the mains…where the brakes are. And then brake gingerly.

The less-obvious issue is water on the runway – water which may not be exactly visible as water during a winter approach. Here- the issue is hydroplaning of the tires on a thin layer of water. Dynamic hydroplaning is related to tire-inflation pressure and a formula exists to estimate the rolling speed at which dynamic hydroplaning is likely to occur.

For an aircraft with a tire pressure of 120 pounds the dynamic hydroplaning speed comes at about 97 knots; touch down at- or above that speed and as little as 0.10 of an inch of water can lift your airplane right off the pavement so that the tires are literally surfing. If that occurs- you’re out of luck with regards to brake action. So what should you do?

• First- use the lowest possible Vref for approach.
• Next- land as close to the runway end as possible to increase available stopping distance.
• Then- don’t grease it in- plant it to break the water’s surface tension and improve tire-to-pavement contact.
• Lower the nose as quickly as possible to get maximum steering control.
• Finally- avoid touching the brakes until below the NASA critical speed for your plane – roughly the square-root of tire pressure multiplied by nine.

It is worth remembering that viscous hydroplaning and reverted rubber hydroplaning can follow dynamic hydroplaning – particularly on smooth pavement patches (oily or rubber-glazed). The results are often the same: loss of braking action and/or loss of directional control. You should remain slow- steady and light on the brakes all the way to the ramp.

On the subject of approaching the ramp- there are few things that throw off the normal appearance of an airport than a coating of snow- with banks of plowed snow lining ramps- runways and taxiways. These piles can hide signage and cover taxiway hold-short lines - or block a crew’s view of other moving aircraft.

Use taxi diagrams- a GPS-driven airport graphic and tower directions when available. There’s no sense surviving the chill of a bad winter flight only to slide off slick pavement and tangle with a snow pile or other airplane because the airport is snow-coated.

Winter’s shallow sun angles- the close proximity of sunrise and sunset- humid conditions and rain on windshields can all contribute to eye tricks with the potential to catch the unwary pilot out. Taking the time needed to prepare properly and to get briefed on destinations can go a long way toward avoiding an over-run- or short-landing because of lighting-conditions that cause illusions.

Finally- flying on a different day- at a different time of day- or to a different airport are often all that is needed to avoid an undesired outcome to a flight. A responsible pilot should never hesitate to pull the plug when the conditions don’t add up to an acceptable risk.

This is where a risk-assessment checklist can be helpful – a grid that compiles conditions into an objective- decision-making tool. For some- that’s a Composite Risk Management tool or other institutionalized set of guidelines.

Either way- with a safety management system or a safety-audit program’s guidelines to hand- the pilot can look at the grid and use safety standards and company policy to make the call with far more ease- there being too many negative factors. Not going there- today- in that weather- to face a non-precision approach… That sort of precision decision-making should warm the heart of everyone flying with that captain.


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