Climb and descent planning
Flying high, however, does call for more extensive climb and descent planning than is needed for a lower cruise altitude.
The first planning issue is climb speed. For a sustained climb, you choose speed based on several factors -- climb phase, ambient temperature, and wind.
Maintain takeoff power and use the best-rate-of-climb airspeed for the initial climb immediately after takeoff to achieve the most altitude gain in the shortest period of time. That's an important safety consideration in the event of a power-loss problem, and it also minimizes the noise impact on residences below.
At 1,000 feet above ground level reduce to climb power and continue climbing at a cruise-climb airspeed. Consult the pilot's operating handbook for the appropriate power setting and airspeed. Cruise-climb airspeed is not an official V-speed, but it does represent a workable compromise between a good rate of climb, reasonable forward progress, good over-the-nose visibility, and adequate engine cooling.
In high ambient temperature conditions you might opt for a lower pitch attitude and a higher indicated airspeed to achieve better cooling airflow through the engine. Conversely, when it's cold you may be able to get to cruising altitude quicker by climbing at a slower indicated airspeed -- but faster rate -- without having to worry about overheating the engine.
If you'll be cruising at altitude with a strong crosswind or headwind component, you might be tempted to maximize the time in climb by climbing at a higher indicated airspeed and slower rate, but this may not be the best choice. The reason is that you'll be holding a greater correction into the wind because of the slower climb airspeed compared to cruise. That means the wind will be taking more of a toll on groundspeed during the climb than in cruise. You might as well make the best of it in high headwind conditions and get to cruise altitude as quickly as possible.
Flying high also means planning for a longer descent. As in a climb, the best descent usually is a compromise between starting down too early and too late.
On one of those rare good days when you have a strong tailwind in cruise, it can be tough to leave that fantastic groundspeed up there and begin a descent to the destination airport. The urge is to delay beginning the descent until the last possible moment. Doing so, however, creates the potential for having to descend too rapidly near the airport to avoid overshooting it, or arriving too high and probably too fast to make a normal, stable final descent and approach.
Avoid slam dunks
The other, more significant problem with rapid descents is the potential for shock-cooling the engine. It can be tempting to yank back the power to idle and point the nose steeply down to descend quickly. But that sudden reduction in power and engine heat, coupled with the increased descent rate and ram-air engine cooling, can prove too much for the cylinders, which can crack.
The converse of a late and too-rapid descent -- beginning the descent too early -- is less problematic, but it may not be the most efficient choice. Long, drawn-out descents from altitude can add to trip time, increase overall fuel consumption, and subject pilot and passengers to extended time in turbulent lower-level air.
So, when's the best time to begin a descent? An easily remembered rule of thumb for descent planning is to multiply the altitude you need to lose in thousands by four, and that's how many minutes it will take to descend at a passenger-pleasing 250 fpm. For example, if you are cruising at 9,500 feet msl and need to descend to 1,500 feet msl, you should begin your descent when 32 minutes from your target altitude waypoint (9.5 minus 1.5 = 8 times 4 = 32). It works for any groundspeed.
Halving the multiplier to 2 halves the time to 16 minutes and doubles the descent rate to 500 fpm; a multiplier of 1 advances the descent rate to 1,000 fpm, regardless of groundspeed.
The formula works well, but you may have to adjust for any number of variables. For example, in strong turbulence you should reduce to maneuvering speed, which obviously will affect descent planning.
Wind is another variable. If you encounter a strong headwind at lower altitudes, you should factor that into your descent planning. Check with flight service prior to beginning your descent for winds at lower altitudes, and use the information to fine-tune your profile.
Working with ATC
Air traffic control is something of a wild card in terms of descent planning unless you know from experience how the controlling ATC facility handles arrivals. Controllers may prefer to descend light aircraft to very low altitudes far from the airport to keep them out of the way of high-performance traffic. One way to deal with inefficient ATC protocol when flying IFR is to negotiate with the controller to cancel your IFR flight plan (assuming, of course, that visual meteorological conditions prevail) but retain your transponder squawk code and continue VFR with ATC traffic advisories. In that case you set the descent agenda.
Keep your passengers in mind when planning a descent. Rapid descents mean a rapid and noticeable increase in pressure. That can generate concern in nervous passengers or even physical pain in someone with a stuffy nose who is having trouble clearing their ears.
Power settings also must be part of your descent planning. Remember that as you descend into increasingly dense air, engine power gradually increases. At minimum you'll have to reduce power at 1,000-foot intervals to avoid exceeding power or engine/propeller rpm limits. You also should enrich the mixture at the beginning of the descent (assuming that you leaned to best power or best economy setting when you first reached cruise altitude), and monitor the mixture setting as you come down to avoid a too-lean setting.
It's important to maintain enough power during descent to prevent overcooling of the engine. If the airplane has separate manifold pressure, propeller rpm, and mixture controls, you should plan your descent so that you reduce power a maximum of two inches of manifold pressure with each adjustment, which should be spaced at two-minute intervals.
In other words, if the manifold pressure is set at 24 inches in cruise, you can reduce to 22 inches to initiate the descent. After two minutes, you can make a further two-inch reduction in power. This is a conservative way to ensure that the engine maintains sufficient heat to prevent shock cooling.
The same general rules concerning power settings apply for an airplane with a fixed-pitch propeller, and throttle and mixture controls only. Reduce engine/propeller rpm initially to begin the descent, enrich the mixture, and continue to make appropriate adjustments as you descend.
You can safely reduce manifold pressure to as low as 20 inches, or engine/propeller rpm to 2,000, in the descent and not worry about shock cooling, even with a rapid descent rate. However, avoid putting yourself in a situation where you must immediately go from a high cruise-power setting to 20 inches or 2,000 rpm and at the same time push the nose down. Sudden low power coupled with a high descent rate is not a recipe for long engine life.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
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