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SAFETY MATTERS - RAIN PRECAUTIONS

March 2010

Category: Safety & Security Issues for Business Aviation

Author: Dave Higdon

Rain Flying:
Up there, it’s about more than wet ramps.

A jet touches down fast and long on the rain-soaked runway, sadly leaving too little pavement ahead to safely stop the plane…

Another jet on a daylight arrival descends below the clouds at a comfortable altitude, but in the transition from Instrument to Visual Meteorological Conditions the crew misjudges their distance from the rain-wetted runway, touching down short…

A turboprop twin maneuvers through light rain in marginal VMC and through the raindrop-cluttered windshield the pilot scans the ground for the runway – ultimately misjudging the distance of the visual approach lights and landing long, stopping barely in time to avoid a runway overrun but blowing out tires in the process…

Finally, a light jet descending in a hard rain begins to slow for the runway, but the jet stalls prematurely and forces the crew into some uncomfortable maneuvering to recover and head out to the missed-approach fix…

Do you notice the common denominator to these different accident and incident scenarios? Rain: nothing exotic – just plain old water. Yet even a little water impacts aircraft and crew… even at relatively light levels of precipitation and accumulation.

While business turbine aircraft as a category are generally well-equipped for all-weather flying, different types of weather nonetheless impose different levels of performance challenges to safe flight – occasionally underlying an outright penalty for the hubris of challenging one of Mother Nature’s most-common of her life-supporting conditions.

Rain in excessive quantities - too much rain, too fast - often overwhelms drainage systems sending torrents of water into streets and onto highways…and runways. But even at low-levels rainfall can rob aircraft of performance, reducing the margins for error – and tricking the aviators’ vision and perception in subtle, insidious ways that are all too human.

At its worst, heavy downpours can denigrate aircraft performance and expose the aircraft and occupants to ancillary conditions with potential for catastrophe. Unfortunately, the best all-weather equipment only helps assure turbine engines continue to run in heavy precipitation, and protects airframe and powerplant inlets from accumulating ice.

This equipment does little-to-nothing to immunize aircraft or aviators from the laws of physics and the penalties of flying through rainfall. In knowing and understanding these issues, pilots, passengers and operators can develop strategies for avoiding the pitfalls of falling water drops – as well as gain an understanding of why sometimes the best solution is a “No-go” flying decision.

‘SOFT’ OR ‘HARD’ IFR
Among pilots a distinction exists within conditions requiring use of Instrument Flight Rules for making any trip. Of course, one involves altitude; the Federal Aviation Administration requires flying under IFR for flights operating above 18,000ft mean sea level (msl) or Flight Level (FL) 180.

For the business turbine aircraft, operating above FL180 seeks simply the best speed and fuel efficiency of the aircraft; flight below that altitude typically involves only the departure and arrival segments of any given flight – or very short legs. Technically, pilots can wait to activate IFR until near FL180 and legally cancel IFR once below… most don’t though; either simply for the extra pair of eyes, or because of company policy – unless some operational benefit exists for that flight, that day.

But on those days when conditions leave pilots with no option – when weather demands an IFR departure and/or an IFR arrival – we tend to view those conditions as either ‘soft’ or ‘hard’ IFR. ‘Soft IFR’ involves usually no more than light precipitation, if any, and benign cloud cover – no convention, electrical discharges or major turbulence, and little to no precipitation.

Meantime, turbulence, from an uncomfortable degree to ‘airplane-bending’ levels, often lurks in ‘Hard IFR’ conditions including lightning storms, thunder, convective weather, icing – even in summer – and, of course, moderate to heavy rain.

We know from experience that business aircraft crew, operators and passengers expect to get to their destination – or back again – in most weather, particularly through anything sounding as benign as Soft IFR. Unfortunately, though, rain can be ruinous in the risks it proffers on even a Soft IFR day because it’s not volume alone that threatens.

We do it; we’ll do it again - but we do so with a deeper appreciation of the impact of rain, regardless of its degree, on an aircraft and crew. Take a look at the reasons (below) for reconsidering a rainy day flight – or for changing practices to deal with that flight.

ENGINES AND AVIONICS
Those complex, expensive powerplants love cool days; the cooler the day (to a point), the better they perform and the happier that airframe is at lifting and cruising. Throw in rain, though, and you may notice some changes…

Moderate to heavy rain can impact engine performance as the intake fan suffers through the erosive nature of even small droplets of water impinging on the blades; in heavy rain, moisture can get past the fan, into the compressor section and eventually into the combustor ring. While the use of continuous ignition may help avoid a flameout from occurring, that much moisture isn’t the best thing for the engine or some of the electronics hanging on it (and inside the nacelle) although most of those components are well protected from moisture.

Rain water can also get into the pressure side of the pitot/static system and induce airspeed errors, which, in turn, can have an impact on flight management system control. Nobody wants the ‘thrill’ of making a final approach with erroneous or questionable airspeed indication – or by having to revert to the standby instruments. Pitot heat can help preclude such a problem from happening, and should be used at the onset of any encounter with rain of any degree.

FUEL: LITTLE WORRY, BUT ENOUGH
Water in any fuel system is problematic; water getting through a turbine-aircraft system and to the engine is rare – but it’s an event that few who’ve experienced it ever want to repeat. So remember the possibility of water getting into the fuel tanks when the rainfall is substantial and the encounter lengthy.

Even fuel vents – necessary for fuel to flow to the engines – can serve as a pathway for water to enter tanks when the rainfall is hard enough and the exposure long enough. If the rain encounter occurs on climb, remember that temperatures fall to well below freezing at the high altitudes business- turbines fly. Cold-soaked fuel can allow water to form into ice crystals that clog intakes to fuel lines leading from the tank to the engine – even with fuel heaters active and anti-ice additive if the water and cold are sufficient.

Avoiding lengthy periods of rain on climb or descent may add a delay to a trip – but can preclude any problems from occurring due to water contamination of the fuel system.

AIRFRAME FOLLIES/WATER-PERMEABLE PARTS
While the fuselage and those areas carrying moisture-sensitive electronics are generally well sealed – pressurization requires such sealing – other parts of the airplane are not so protected. Water can, and will accumulate in flap sections, ailerons, elevators, rudders and even vertical and horizontal stabilizers.

If the airplane has been sitting on a ramp exposed to rain, the pre-flight check should include a little extra attention to those surfaces and draining water; otherwise, those freezing temperatures up in the Flight Levels can turn liquid into ice – with results no one wants to learn the hard way.

Some of these surfaces may accumulate water in-flight which - on freezing - results in an imbalance of control surfaces. Be mindful of this possibility when the rain is particularly heavy, the exposure particularly lengthy, or both.

ROLLER-COASTER FOLLIES
Flying through even light rain usually means experiencing a rougher, less-comfortable flight – and the harder the rain, the less-comfortable the turbulence. Experienced pilots know that systems with rain often hide unstable conditions primed to make the day less fun and the air-sick bags more popular.

Approaching the boundary areas of rain-carrying systems ups the odds of encountering downward moving air and the feeling that the bottom has fallen out from under the aircraft. Microbursts – air falling so fast that it can deprive the wing of lifting airspeed – can occur down near the runway, up at cruise altitude, or in between.

Encountered close to the ground – such as flying into a hard rain while below 1,000 feet agl on final or on departure – can make a flight nearly uncontrollable, and put the aircraft at risk for ground impact. “Controlled Flight Into Terrain” (CFIT) is the result of too many such encounters. When the microburst encounter occurs at altitude the falling air can drag the aircraft down rapidly through altitudes used by aircraft going in other directions.

THE AERODYNAMIC PENALTIES
So far we’ve addressed the problems rain can cause by invading sensitive wiring, impinging on engine performance, airframe parts and the stability of the air itself. But in its final assault on aviation, rain also impinges on the aerodynamic performance of the aircraft’s lifting and control surfaces.

Light rain, for example, can contaminate the airflow closest to the wing, creating a mist of air-water mixture that slows the airflow and reduces lifting power of a wing or tail surface.

Moderate and heavy rain can create a thin layer of liquid water on lifting surfaces, changing their character in ways that both reduces lift and increases drag. Raindrops striking that layer of water roughen it with dynamic dimples of moisture, further reducing the lifting power while raising the drag profile of that surface.

According to research recognized by the Transportation Research Board, the effect increases with angle of attack, becoming its most severe at high angles of attack and after deployment of flaps or slats. The decline in lift can go as high as 18 percent while the increase in drag can soar by 40 percent. The result of these two aerodynamic hits: a greatly increased stall speed.

That stall speed – in even the lightest rain, it’s worth noting – can go up by as much as 17 percent, while the angle of attack at which stall can occur declines by as much as half to as little as nine degrees AOA. These subtle, but substantial changes can merge to produce deadly consequences.

THE COMPOUND EFFECT OF RAIN
Now, let’s put together these penalties and changes, small and large. Match up deteriorated aerodynamics, a higher stall speed, lower stall angle, and an otherwise normal approach through light rain. It’s not hard to imagine the stall happening, quite innocently, too soon, too far away and too high to safely handle – and too low to recover from. Throw in a microburst (even a tiny one) and all bets are off. Either way, the crew can find themselves on the ground well before planned – and more than a bit short of their intended touchdown point.

Compound the aerodynamic issues with the slower speeds and higher stall speeds of maneuvering for a runway, and the prospect greatly increases for rain and turbulence to take their toll on the unenlightened aviator. Flying a higher approach speed to increase your safety margins impacts runway performance – something to deal with before turning final - so knowing there are issues to deal with, what they are and how to deal with them can go a long way to minimizing the mayhem.

GETTING OUT OF THE RAIN PROBLEM
Of course, the first and best decision for countering the threats of a rain encounter involves the simple decision to wait: Sitting out a problem never turned it into anything regrettable where aviation is concerned. A different route, a later or earlier arrival time, maybe even a different day can turn a potentially threatening day into a non-issue.

But some days it seems that no option exists - some days the bad weather occurs in spite of a forecast for better; and some days you’ll fly regardless, because it’s “just a little rain.” First, as you should now see, even “just a little rain” can be a big deal. That said there are strategies and techniques for surviving the decision to go anyway. But they aren’t all intuitive.

For example, the first sign of a microburst encounter is usually the sense of the airplane falling. The most-common, instinctive response typically involves pitching the plane up to best-rate-of-climb angle and using full engine power. When encountered in a heavy rain, however, the microburst really calls for a pitch down to recover flying speed, adding power and smoothly pitching back up to gain a positive rate of climb – to the minimum needed to establish climb.

Remember that a wet wing stalls at a higher speed and lower angle of attack – so first pitching up to recover from a rainy microburst will put the airplane well below stall speed and well above best AOA – not a good scenario for accomplishing recovery. Remember, too, to keep the speed you seek about 20 percent above stall for your configuration, to better avoid an inadvertent stall close to the ground.

Speaking of close to the ground, recovering from a microburst at or below pattern altitude for an airport can be unsettling – it’s counterintuitive to push down so close to the ground. But a little practice at more-comfortable altitudes can help alleviate the discomfort and build confidence in the technique.

HUMAN FACTORS
The issue of flying in the rain comes with its own set of human factors - elements beyond the issues it brings to airframes, aerodynamics and engines. And these human issues are as important to understand in the safety of operations as the aircraft considerations.

The biggest issue for humans involves our vision and perception and is well documented in an article published in India’s Naval Aviation Journal, Meatball. First, flying in haze: Haze gives our vision the impression that a runway is farther away than it actually is, which can prompt the pilot to shallow up the glide path.

Second, when flying in light to moderate rainfall, runway features take on a softer look and become less distinct due to a halo effect precipitation creates. That effect can increase the chances of misperception of the horizontal or vertical displacement from the runway end when the pilot goes visual after flying on the gauges; improper correcting or over correcting can lead to other problems.

Third, heavy rain exacerbates both distance and depth perception due to water contamination on the windscreen which often leads the pilot to perceive the aircraft too high; the push down can lead to landing fast or falling below the glide path with a short-touchdown possible.

Fourth, rain impacts how we see approach lighting. During the day, rain dims the intensity of the approach lights, which makes them appear farther away, resulting in an effort to shallow the glide path to get there – and a long landing, as a result. At night, rain makes approach lights seem brighter and closer than they actually are, setting up a potential short-landing scenario.

Finally, a wet runway can produce an illusion that the pavement is farther away than it actually is because the surface doesn’t reflect light back to our eyes.

OTHER CONSIDERATIONS
There is one final set of considerations worth remembering even after the rain stops… that wet runway ahead. It may carry standing water; if not properly grooved, it can have water flowing over it in sheets – all problems for braking action on touchdown, even after the rain stops and you make an otherwise- normal approach.

Couple these wet runway issues with continuing rainfall, and the problems compound. For example, knowing the threat to stall speed and the higher approach speeds warranted in rain, light to heavy, can produce the wise decision to up the approach speed beyond the usual bad-weather level.

The downside of this comes when the tires touch the wet runway – which boasts far lower friction than when dry. If the runway is sufficiently long, a too-fast touchdown doesn’t threaten much; make the runway “just-long-enough”, and circumstances change to “not-long-enough” for that higher touchdown speed. Even issues like a down-slope runway can further complicate the issue.

At the end of it all, the issue comes down to one of judgment and airmanship. If, in the judgment of that professional flight crew, the risks of departing in, flying through or arriving with rainfall exceed the prospects of safe arrival, it’s a wise passenger, owner or operator who listens, heeds, and thanks that crew.

As this shows, even a little rain can raise risks disproportionately – and knowing these issues is why we hire the best pilots we can find. Ignore their judgment at your peril, with or without your umbrella.

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