Do you want to fly high or low when facing an adverse weather situation? There are no hard and fast rules, of course. The weather's quixotic nature, the mission at hand, your experience and comfort level as a pilot, the attributes of the airplane you're flying, and the special conditions that attend every meteorological event see to that. However, there are some general guidelines that can provide a basis for the beginnings of your internal debate — which, by the way, should come well before takeoff.
First things first. Right up front, let's acknowledge the advantages of flying at higher altitudes. One big benefit is higher true airspeeds. On average, and depending heavily upon density altitude, gross weight, and other variables, you'll realize a two-percent increase in true airspeed (and hopefully, groundspeed) with each 1,000 feet of altitude. So an airplane with a maximum cruise true airspeed of 130 knots at 3,000 feet msl will make about 143 knots up at 8,000 feet — if it can maintain the same power setting at the higher altitude. (Most normally aspirated engines begin to lose the ability to put out 75-percent power at around 6,000 to 8,000 feet msl).
Turbocharged engines permit high-power operations all the way up into the oxygen altitudes. Eventually, though, the turbo's wastegate will close and its power will drop off like that of a normally aspirated engine. Still, turbocharging can usually let you cruise at altitudes as high as 25,000 feet, where the true airspeed advantage really kicks in. As for jets, the rule of thumb is to climb as fast as you can to cruise altitudes as high as you can manage. As with piston-powered airplanes, this strategy both maximizes true airspeeds and minimizes fuel consumption. But with jets, these benefits are reaped in spades.
This raises yet another important point. Fuel savings are another big advantage of cruising at higher altitudes. The higher you fly, the more the fuel-air mixture must be leaned. This reduces fuel consumption and extends range at the same time. The only caveat here is to make sure that your leaning is not done too aggressively. The thin air at altitude may permit lean engine operations, all right, but it also does a lousy job of engine cooling. In warmer-than-standard temperatures and/or higher altitudes, special care may have to be taken to keep the engine's temperatures within acceptable limits, and this can involve enriching the mixture, opening cowl flaps, changing the power setting, or all of the above.
If high power output isn't an objective in your flight plan, then fuel savings can be significant when flying at higher altitudes. For example, take a Cessna 172 flying at 2,500 rpm — at 2,000 feet. Under standard atmospheric conditions you'd be running at 76-percent power and burn about 8.5 gph — that's with the mixture leaned to 50 degrees Fahrenheit rich of peak EGT, the recommended setting. With this setup and the Skyhawk's 50- gallon long-range fuel tanks, your range with VFR reserves is about 575 nm and your endurance is just over five hours. True airspeed should be 114 knots, according to the POH.
Now take that same airplane up to 8,000 feet and cut the power back to 2,400 rpm. Here, you're making just 60-percent power. But in spite of the power drop, your true airspeed is 106 knots — a drop of just eight knots from the speed way down low. Range and endurance, however, are ratcheted up to 660 nm and five hours, 24 minutes, respectively. At the same time, fuel burn is cut to 6.7 gph, almost two gph less than the fuel flow at 2,000 feet.
Increased glide range, improved communications and navaid performance over VHF frequencies, and better terrain clearance round out the very impressive list of high-flying's payoffs.
Do all these perks translate into good weather flying strategy? In many cases, flying high can make a lot of sense.
Selecting a higher cruise altitude can let you top cloud layers, for one thing. In winter, this can mean avoidance of icing conditions. In summer, you'll experience less turbulence when flying above the bumpy, convectively charged layer of air nearest the surface. Visibilities will be better "on top," also. This improvement in visibility is particularly noticeable in the summer months, when temperature inversions can trap pollutants in a low-lying (and not-so-low-lying, as the summer months progress and haze layers extend to 10,000 feet msl or more) blanket of brownish murk. These inversions are worst when strong high pressure prevails for several days; persistent Bermuda highs are famous for producing IFR visibilities in haze.
Flying above clouds, haze, or any other obscuration carries some risk for non-instrument- rated pilots. VFR flight on top of a cloud deck is perfectly legal but may not rank as the brightest of ideas. A scattered or broken layer below can quickly turn into a solid undercast, leaving the VFR-only pilot to face a descent on instruments and, if the fates are frowning, poor visibilities, high terrain, an uncertain approach, or all three. The best rule is to make sure that the breaks in the clouds below are large enough to permit a VFR descent to the destination. In other words, conform to visual flight rules.
The down side of flying high mainly involves winds aloft. Usually, the higher you fly, the stronger the winds. That's great if they're tailwinds, but stiff headwinds can mean more than a longer trip. Fuel consumption is increased with additional time spent in cruise, and range is reduced. Can you, or should you, tolerate these effects? Yes, if topping turbulence or a layer of ice-laden clouds is on your list of priorities and if the trip is to be a short one. No, if fuel reserves are limited, terrain is a factor, or suitable airports (for precautionary landings, in case of diminished fuel reserves or worsening weather) are few and far between.
Topping cloud layers can also be a successful means of avoiding thunderstorms or other cumulus buildups. Keeping a safe distance from convective activity should be every pilot's objective, instrument-rated or not. The best way to do that is to keep an eye on menacing clouds, and that won't happen if you're slogging along on instruments or weaving your way around low-level clouds, trying to remain VFR. Better to be above clouds, able to spot any telltale signs of dark clouds or monster cumulus well before they affect your route of flight.
With all these advantages to flying high, does it ever make sense to fly at lower altitudes? Absolutely. In those cases where daylight, scattered rainshowers, and safe ceilings prevail, flying beneath a cloud layer can be an excellent means of traversing an area of adverse weather. This assumes, of course, that terrain — and airspace restrictions — are not factors, and that the rain showers are just that, and not the product of thunderstorm activity. Shafts of rain can make areas of precipitation easily visible and easier to avoid. That's a general rule, of course. Lighter concentrations of rainfall are often invisible and can catch you by surprise; fortunately, these types of showers don't often affect flight visibility. (Big, big caveat: these types of showers can contain freezing rain if the outside air temperature is right — especially in warm front situations).
From days of yore comes the advice to fly low as a means of circumnavigating thunderstorms. Steer toward the light areas, say the old-timers, and fly around the rain. While this strategy may work in specific instances, there's simply no reason to apply it as a general rule. The fly-low- go-to-the-light theory may be fine for Florida afternoons or the desert Southwest when no lines or clusters of thunderstorms are present, but it won't work everywhere all of the time for several good reasons:
- Flying beneath a thunderstorm can be just as harrowing as penetrating one on instruments. You may be in the clear for now, but how about the next minute? Airplanes have been sucked into storm cell cloud bases by powerful updrafts, tossed around by severe turbulence, and spit out in pieces this way.
- "Light areas" may turn ugly. If the weather's convective, it's changeable. Trying to navigate visually can be disastrous if you bank on a less-dark piece of sky as your only out.
- Clouds can close in all around you. For this reason, even a 180-degree turn may not work in really extensive areas of convection.
- The only really safe answer? Avoidance of thunderstorms and their associated clouds by margins adequate to allow visual separation by at least 20 nm.
But if flying low may not always work as a thunderstorm avoidance technique, it very often helps a great deal in achieving higher groundspeeds. Down low, normally aspirated piston engines develop more power, winds aloft are generally — generally — less intense, and so you'll tool along at a better clip. Since going fast is a reason most pilots fly in the first place, going low is the method of choice for many of us. You may get bounced around and have reduced visibilities, but at least you'll be flying at or near your airplane's optimum true airspeed and, hopefully, groundspeed.
Truth to tell, most piston pilots are so accustomed to flying at lower altitudes that there's seldom a thought about cruising much above 7,000 or 8,000 feet msl except when flying in mountainous regions. Perhaps this is a holdover from our early flying days as student pilots, when the focus was on pattern work and other low- altitude drills. Then, 5,000 feet seemed like an astronomical height, an altitude reserved only for long solo cross-country flights.
So which is better, cruising high or low? It seems that higher altitudes earn the most ayes. But with strong headwinds, fair weather, flat terrain, and shorter trips, flying low may well be the way to go.
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