Altitude Management

Altitude Management

Altitude Management

Basic airmanship obviously requires management of the third dimension of mobility, that of height, in addition to forward speed and left-right movements. An aircraft’s ability to rise and descend means we must plan our actions to reach a point in space defined by distance in a vertical plane, not just map coordinates.
This seems simple enough at first, but, given the requirements of avoiding cataclysmic contact between aerial vehicles, as well the undulating surface of the earth, complexity arises. Altitude management takes some advance planning, not reaction to a developing situation. The traffic flow may require beginning and halting altitude changes at specific times and points, as planned and as directed.
Therefore, we not only need to know what altitude is required to achieve our objective of safe flight, we need to know when it’s okay to descend, when or where we can start the climb, and where the greatest efficiency can be obtained. Choosing, maintaining and changing an altitude isn’t always simple.
What altitude shall we ask for? Almost always, we’re after comfort first, then efficiency. I asked an acquaintance about the long-range cruise power setting for his Cessna C510 Mustang. He said “FL 390 or 410.” Unless chop is present at those altitudes, that’s where his little Pratt & Whitneys do their best work, on the least fuel burn. Flying lower will often be required, but range will suffer.
As high-flyers soon learn, it can sometimes take twice as long to fly a route westbound as it will to return eastbound. One must balance fuel efficiency against ground speed, and plan contingency stops in case the winds do not improve, bearing in mind that descending for fuel and climbing back to altitude is costly. Modern FMS equipment can show “fuel remaining on arrival” almost instantly, and it’s nice to be able to see the results of our efforts to “make fuel” by changing to a more favorable altitude. One can’t always be successful in this endeavor, however, so having a solid-gold alternate for a fuel stop is important.
Just about every airplane has its “best altitudes”, like the Cessna Mustang I referenced above. For normally-aspirated piston planes, it will be about 7,000 feet MSL. For those with turbocharged engines, the sweet spot will come at the critical altitude for maximum cruise power, where the turbo’s wastegate is closed in order to maintain cruising manifold pressure. Further ascent results in a reduction in power output, because the turbo is already working as hard as it can. Most often, this occurs at around 16,000 to 17,000 feet.
Turboprop engines are normally aspirated, producing less power as one ascends, so the best altitude is one at which fuel efficiency, which improves with altitude, meets the highest true airspeed, obtained in the thin air before power loss at altitude becomes too great. Engine makers can “turbocharge” a turboprop by increasing compressor capacity and installing a more-robust turbine wheel that can withstand higher temperatures, then restrict sea-level horsepower to the maximum the airframe designer had in mind. The “flat-rated” excess power can be used to maintain sea-level output to higher altitudes, so it will be possible to fly faster up high, or fly at the same speed with less fuel burn.
Jets are heat engines, and will do their best work in the cold climes of high altitude, where airframe efficiency is best and the ratio between intake and exhaust temperature is greatest. The “best altitude” may be restricted by compressibility concerns as design Mach limit is reached at a slower TAS in the cold air. Aerodynamic stall meets Mach buffet at “coffin corner”, at an altitude where maximum fuel efficiency may be found. With jets, it’s best to fly as high as you can for as long as you can, and to climb as high as you can as fast as you can. As I was told in a recent conversation on the subject, “you can climb 10,000 feet for each 10 minutes of level cruise flight.”
All of these desires, of course, are subject to the whims of air traffic control, which can’t always accommodate a bunch of aircraft wanting to go the same way at the same altitude. Convective weather poking up into our most-desired flight levels wreaks havoc with the best-laid plans.
Precision flying requires maintenance of an exact altitude as well as a route’s track and heading. Despite the inherent flaws in referencing a pressure plane that sometimes changes, and altimeters that may possess tens of feet of error, there’s a certain satisfaction in holding a specific assigned altitude. In airspace designated for required minimum navigation performance, the tolerance for deviation may be small. Most controllers will call if your reported altitude is more than 200 feet off the mark. Transponders report each 100-foot of altitude at the mid-point of each increment, so being 250 feet off will show up as 300 feet.
Passengers Also Need Love
Altitude management must always be conducted with regard for the passengers’ comfort and safety. While ATC protocol is based on a minimum climb/descent rate of 500 fpm when directed to vacate an altitude, we don’t want to throw passengers against their seat belts (assuming they have them on) or spill drinks. In most cases, we’ll be on autopilot and will direct flight-level change through the AFCS. Start with a click or nudge on the knob, to lower or raise the nose a bit, then add another couple of increments after the nose begins to move, inputting more attitude change in gentle advances. Power movements may also be subtly managed to avoid sudden changes in the noise level. There should be no perceptible G-load imposed on the cabin’s occupants. You might like flying a fighter-plane, but they won’t.
Positional awareness is certainly vital to the job of managing altitude, most particularly during arrivals and departures. A procedural altitude is only correct when it’s verified by charted references. TAWS and synthetic vision warnings notwithstanding, the task of altitude management should be to prevent these tools from being activated, by knowing where we are and when we can go into relaxed-mode, safely above all hazards. Even in the flat country, towers can rear up 1,000 to 2,000 feet above the terrain; one airport I frequent, usually cleared for a visual arrival, is ringed by these too-tall spires of steel, and I caution pilots unfamiliar with the area to limit descent until established “in the slot” on final.
When Things Go Bad
Planning for abnormal or emergency conditions requires that you know where you’re going, altitude-wise as well as laterally. If you lose an engine on a high-and-hot departure, you should know the level of climb performance to expect, and which way to turn if you find yourself lacking in capability. Climb gradient in feet per nautical mile can be roughly computed, using the two miles per minute of the common 120 knots Vyse of many airplanes. Thus, a required 200-feet-per mile gradient takes 400 fpm to meet, exclusive of wind. I can assure you, what appears to be possible on paper looks very intimidating from the cockpit, as one struggles to gain altitude with an ill airplane. The slightest downdraft negates all one’s efforts.
If you’re flying a single-engine aircraft, or a normally-aspirated piston twin near its single-engine ceiling, your contingency planning needs to be based on the inability to hold altitude after losing an engine. Always know the nearest landing facility and the heading to the safest, lowest terrain. There are always options, but you mustn’t let them get out of reach.
Managing altitude is a piloting task that affects most of our other flight planning; range, endurance, obstacle clearance, and ability to reach the expected destination. Give it its proper due.

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