Summer’s seen—and sometimes unseen—hazards

1710
image: Chad Evans

Summertime flying is usually quite attractive, but it brings a number of considerations for pilots. Several are mentioned frequently enough that pilots know about them. Others get less attention in training and in the press, and as a result pose a greater risk to pilots and their passengers. Let’s review summer’s seen—and sometimes unseen—flying hazards.

Heat

Almost everything about powered flight is affected by air density. As air density decreases, that is, as density altitude goes up, less-dense air means less potential for power development in normally aspirated aeroplanes—this is why most piston engines must be leaned for high-density altitude take-offs. Turbocharged and turbonormalised engines are affected too, because their turbos have to work harder to make up manifold pressure loss, and in doing so they compress the air further, heating it more. Even turbine engines are affected, again because of the heat of compression. Whether normally aspirated or turbo’d, piston or turboprop, propellers are less efficient turning power into thrust in thinner air.

When air molecules are less densely packed, wings must travel a greater distance in a given amount of time to impact enough air to generate needed lift. True airspeeds are higher in hotter air; take-off and landing distances increase.

So, heat plays a big part on power and aeroplane performance, most critically during take-off, initial climb and landing. Density altitude is a common topic of discussion each year as the temperature rises, and we all learn how to predict performance using handbook charts. But there are a couple of unseen hazards, that is, factors not as frequently part of training and pilot talk, as well.

The first is convective turbulence. As sunlight strikes the earth, dark areas such as car parks and large buildings absorb solar radiation. This heats the surface—heat radiates to the air above, and hot air rises. So, on sunny days, there are updrafts above dark-coloured surfaces. Lighter-coloured surfaces, conversely, reflect the sunlight. Trees, grasslands and bodies of water do not heat as much, so the air above these types of surfaces is cooler and descends. At cruise altitude these differences are not usually noticeable. During take-off and landing, however, flying through successive areas of rising and descending air causes turbulence—and not often taught, momentary reductions in climb performance and deviations from glidepath during landing (see figure 1). We have no Pilot’s Operating Handbook charts to help us predict how much performance may be degraded as a result of convection in addition to density altitude’s impact. We just have to give ourselves an extra-wide margin for safety for hot-day airport operations and obstacle-avoidance climbs.

Figure 1: Convective turbulence impact on glide path

Even less commonly known, in the heat of summer, computed density altitude is only part of the hazard for take-off. The air in the first couple of metres above a dark-coloured runway can be as much as 20°C hotter than the reported temperature. That’s the air your engine and wing are using to generate power, thrust and lift, so performance may be significantly less than even density altitude calculations suggest. If the ambient temperature is about 30°C or more, the sun is shining and the winds are fairly light, add 10 to 20 degrees to the ambient temperature in your calculations and see if runway surface temperature performance is still sufficient for you to fly.

Humidity

Humidity’s impact is not nearly as great as temperature, but when the air’s hot and humid there’s an even great degradation in performance. Molecules of water vapor are much larger than oxygen atoms alone, so humidity displaces oxygen in a given parcel of air. Engines will develop a little less power, propellers turn that power into a little less thrust, and wings generate a little less lift. Aeroplane flight manuals don’t provide any guidance on how to account for humidity, but as a rule add another 10 per cent to runway and obstacle clearance distances when the relative humidity is greater than about 60 per cent.

Paradoxically (as CASA puts it), when the relative humidity is high carburetor ice is likely at temperatures as high as 38°C—summertime flying. Especially at reduced throttle settings, when the intake manifold is already partially closed by the throttle, carburetor ice can block the remaining intake air flow and cause power loss. If you fly an aeroplane with a carbureted engine, follow the manufacturer’s recommendations for applying carburetor heat. If the handbook isn’t clear on the topic, apply carb heat fully any time you reduce power for glide or in the airport circuit. Figure 2 defines the carburetor icing hazard as a function of temperature and humidity.

Figure 2: Carburetor icing potential

Thunderstorms

The most dramatic of warm-weather flying hazards, thunderstorms are primarily dangerous because of strong turbulence in, around, and under the thunderstorm clouds. Hail and airframe ice are lesser but still significant threats; the most obvious characteristic of thunderstorms, lightning, is the least of their hazards. Yes, we know to stay clear of thunderstorms, even that we should stay about 30 km or so from storms to avoid the threat. With the advent of weather data uplinks to the cockpit we have far better information for avoidance. But what is and is not acceptable for safely avoiding storms is not what many of us believe. Here’s why:

Radar and the life cycle of a thunderstorm

It’s easy to look at the greens and yellows and reds on an uplink display and think, ‘As long as I stay out of the yellows and reds, I’m okay.’ After all, the greens often indicate very light precipitation. Significant turbulence, however, is often found outside the yellow and red areas. This is because radar does not indicate the earliest but still dangerous phase of a thunderstorm’s development.

The three stages of a thunderstorm’s development are the cumulus, or updraft stage; the mature phase; and the downdraft or dissipating stage (see figure 3). Radar does not detect turbulence, it displays the precipitation that sometimes (but not always) coincides with areas of strong turbulence. The mature stage of a thunderstorm is defined as starting when precipitation begins to fall from the cloud. But it’s turbulence, not precipitation, that is the hazard of flying near a thunderstorm. By definition, the updraft stage of thunderstorm development is invisible to radar despite the potentially damaging wind shear it contains—as soon as the storm appears on radar it is no longer in the updraft stage.

Figure 3: Life cycle of a thunderstorm cell. In the cumulus, or updraft stage, vertical air speeds of 2000 feet per minute can occur. The mature stage, defined as when precipitation begins to fall (and therefore the first echoes appear on radar), has rising and falling air currents on the order of 5000 to 6000 feet per minute. In the dissipating (downdraft) stage, air descends at 6000 feet per minute or more. It’s encountering the wind shear between still air, rising and falling columns of air that is the major hazard of thunderstorms. An individual cell in an area of thunderstorms can complete the entire life cycle in about 30 minutes from beginning to end.

30 km from where?

The old advice about avoiding thunderstorm activity—to remain 30 km from the edges of any storm cell’s radar return, and to stay in visual conditions if at all possible while doing so—is just as valid now as it was in the days before weather data uplinks and moving map displays. The true purpose of these technologies is not to help us penetrate areas of precipitation returns, it’s to make it easier for us to stay 30 km or more away.

In the 1960s Dr David Strahle pioneered digital transmission of weather radar information into the cockpit. As such he is known as ‘the father of datalink radar’. He emphasises that it’s generally safe to fly through areas of light precipitation (‘green’ returns on most radar plots), if there is no moderate or greater precipitation associated with those clouds. However, Dr Strahle tells us, if there is any moderate precipitation in the radar plot (generally yellow), you need to remain at least 15 km away from even the light (green) returns that surround the heavier precipitation. If there is heavy (often, but not always orange or red) or extreme (darker red, white or other) precipitation, remain at least 30 km away from even the light (green) returns (see figure 4).

Figure 4: Flying in an area that looks like this? Avoid all the radar returns … even the green.

Why is this? Thunderstorm research shows that individual storm cells will ‘share’ or ‘exchange’ energy, creating massive areas of instability and turbulence between them that may be invisible to radar … and even to the eye. If there is any precipitation at all in an area of storms with moderate or greater precipitation returns anywhere in the cluster, researchers tell us, there is the chance of extreme turbulence hazard. Dr Strahle warns that if a complex of thunderstorms has enough energy to create yellow or red radar returns, then it has enough potential to create turbulence anywhere within or near the cloud.

So what exactly should you stay 30 km away from? If there’s moderate, heavy or extreme precipitation in the cell at all (yellows, reds or worse), it’s not safe to be anywhere in the precipitation footprint of that cell. Remaining 30 km clear of that thunderstorm means staying at least that away from the outside edges of even the lightest, green radar returns.

Seen and unseen hazards

To mitigate the hazards of summer flying:

  • Operate the aeroplane as light as safely possible. This maximises performance on the available power. Note that discretionary aeroplane weight is usually a function of fuel load. New CASA requirements for fuel planning apply, and if unforeseen circumstances require you to use part of your required fuel reserve you are required to declare a fuel mayday. In hot weather you may have to make shorter flights and refuel more often to get acceptable take-off performance and still arrive within a safe and legal fuel reserve.
  • Apply a significant safety margin to computed take-off and climb performance. Unseen hazards like runway surface temperature, high humidity and convective turbulence during climbout can greatly diminish actual performance even more than density altitude computations suggest.
  • With carbureted engines, check the carb icing potential before flight, and apply carburetor heat as recommended by the aeroplane’s flight manual or any time you reduce throttle below normal cruise settings to prevent the formation of carburetor ice.
  • Anticipate the airspeed and glidepath deviations that will result from convective turbulence over light- and dark-coloured surfaces on final approach.

Summer is an inviting time to fly. Be prepared for its seen and unseen hazards.

4 COMMENTS

  1. Include in your thoughts, landing towards the west in the evening or east in the morning can leave you partially blinded by sun glare. Remember on a nil wind day that a bluster from behind could upset your take off distance.

  2. Yep ..even my near sea level airport often has a density altitude of 2000 feet on a hot day which can degrade the performance of your aircraft.
    btw ..do you still remember how to work out density altitude.? If not, there are some on line density altitude calculators that can make the task simple. QNH and dew point temps which indicate humidity can be gleaned from various weather sites.

  3. A rule of thumb for DA is double what yr field elevation is on anything over 30Degs C. if it’s marginal with those numbers in yr perf book then seek out accurate figures BUT remember ALL the figures in ALL manuals are for new planes, new engines flown by experienced pilots under controlled conditions! Your tired old Cessna, with a barely marginal prop & a poorly kept airframe will NOT perform book figures ! Also remember a FP means you will NOT get rated HP at T/off even at SL!

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