Turbine Pilot

Gettin' down in a turbine

Newcomers to turbine aircraft sometimes feel a little overwhelmed the first time they get behind the controls. It isn't that jets or turboprops are inherently difficult to fly, because they aren't. With proper training, most reasonably experienced pilots can transition to the turbine set. Often, though, the aircraft's greatly expanded performance envelope takes some getting used to. What was normal cruise speed in a fixed-gear, single-engine recip becomes rotation speed in a jet. Weather that would have grounded a less capable aircraft is simply part of the scenery drifting past far below. Bladder-bursting half-day trips are flown in far less time — and sometimes there's even a lav on board.

While coming to grips with this new world order, descent planning invariably looms as a source of angst for first-time flight level fliers. What appeared a simple matter from 5,000 feet — how to arrive in the airport traffic pattern at the proper altitude and location — can seem to require advanced calculus from FL290. When hurricane force tailwinds blow at altitude and a headwind reigns at the destination, for instance, normally savvy pilots are chagrined to discover they don't know when to start down. After all, it is embarrassing to descend excessively early or, worse yet, grossly late for the approach. But most pilots adapt easily to the faster, farther, and higher realities of turbine flying and soon discover how to make descent planning easy again. This mind-expanding realization usually occurs around the time they learn a simple rule of thumb known as the "three to one" rule.

The rule works well for many jet and turboprop aircraft, and utilizes a ratio of three miles across the ground for each thousand feet of desired altitude loss. For example, the pilot of a jet cruising at FL350 who wishes to descend to a sea level airport simply multiplies the number of thousands of feet to be lost — 35 in this case — by three. The result, 105, is the number of miles from the airport where the descent should be initiated. Jet aircraft typically descend at idle or near idle power, with vertical speeds of around 2,500 to 3,500 fpm. At these descent rates, jets can maintain cruise mach, and at lower altitudes, cruise indicated airspeed without requiring additional thrust above idle. When a slower speed is needed, such as 250 knots passing below 10,000 feet, the descent rate is shallowed to around 1,500 fpm (still at idle thrust) to bleed off excess speed. With skillful speed and energy management, the pilot in the example above would leave the power at idle all the way from FL350 until extending gear and approach flaps near the airport. Of course, this presumes an ideal world — without ATC delaying vectors, assigned speeds, or other flys in the descent planning ointment.

Turboprop pilots use a variation of the same 3:1 rule. Unlike jets, though, idle power descents are not normally made for a couple of reasons. First, idle power settings in some turboprops result in flat propeller blade pitch. This, in turn, produces high drag and requires uncomfortably steep angles of descent to maintain airspeed. Secondly, in some aircraft there is insufficient engine bleed air available at idle power to maintain cabin pressurization. Thus, power-on descents tend to be the norm with turboprops. Shock cooling, by the way, is not a consideration. The hot section of a turboprop engine, like that of a pure jet engine, remains at a relatively constant internal temperature when reduced to idle, unlike the cylinders of a reciprocating engine.

Consider several points about this technique for zero-wind situations where true airspeed is equal to groundspeed. First, it works over the very wide range of ground speeds that a jet or turboprop experiences in a descent. At the top of a descent, for instance, a jet aircraft's true airspeed is quite high, 440 knots or so being pretty typical. Passing through 10,000 feet, when the aircraft is slowed to an indicated airspeed of 250 knots, true airspeed has dropped to around 290 knots. It continues to decrease until touchdown. Thus, the 3:1 ratio of ground covered to altitude lost is an average, not a constant relationship through the descent. The actual descent rates required to maintain the ratio will, of course, depend upon the aircraft's average groundspeed during the descent. Normally it will be in the 1,200 to 1,700 fpm range initially for turboprops, and about double that for jets. According to Jeff Titus, Beech King Air program co-ordinator at SimCom Training Centers, a normal descent rate in a Beech King Air C90 is 1,500 fpm, or about half that of a typical jet. But since its true airspeed at altitude is also roughly half that of many jets, the 3:1 ratio still works for descent planning.

Secondly, the rule is more a gauge than a precise calculation, and in the real world this turns out to be a good thing. In light wind situations, it tends to be conservative. If anything, a pilot using it will probably satisfy an altitude restriction a little early, rather than a little late. The FAA doesn't hand out prizes for exactly meeting a crossing restriction, but has been known to give violations for missing them. Being a little early is therefore good. (Consistently early descents exact a cost penalty, though. A joint NASA/United Airlines study a few years back concluded United would save $616,000 a year in fuel costs if each pilot increased his descent planning accuracy by an average of just one mile. But unless one's annual fuel bill is measured in the billions, similar savings aren't in the cards.) In any case, by monitoring the descent carefully, it is easy for a pilot to make small adjustments in the descent rate so that level-off occurs very near to the desired point.

Of course, winds are never really calm during a high-altitude descent. The 3:1 rule works well for jet aircraft when the wind component at altitude is less than 40 knots or so. With much more than that, it requires modification. Rather than change the usual descent rates, though, most pilots find it easier to alter the point at which they begin descent. How much to adjust is a matter of practice with the particular aircraft one flies. Ten miles sooner or later is a good starting point in a jet when the tailwind or headwind component approaches 75 knots. With strong winter jetstreams of 150 knots or better, a 20- mile-or-more adjustment may be appropriate.

Compared with a jet, a slower moving turboprop experiences more net effect from the same wind. It spends more time in the wind than would a jet flying the same route, and the wind represents a greater percentage of its true airspeed. Therefore, its pilot must compensate more aggressively for strong winds.

To illustrate this point, we asked Houston-based Universal Weather and Aviation, Inc., to prepare several flight plans. The first was for a Lear 35, flying from Port Columbus International Airport in Columbus, Ohio, to Norfolk International Airport in Norfolk, Virginia, with a 100- knot tailwind at altitude. The Lear's long-range cruise true airspeed was 370 knots at FL370. Combined with the tailwind, this resulted in a cruise groundspeed of 470 knots, 27 percent faster than true airspeed. Universal's computer programmed the descent to begin 101 miles from ORF. Note that this happens to fit the 3:1 rule nicely: 37 x 3 = 111 miles. 111 miles minus 10 miles (to compensate for the 100 knot tailwind) = 101 miles.

The second flight plan was for a King Air C90 flying the same route at FL250. It too enjoyed a 100-knot tailwind at altitude. But because it was flown at a slower 200-knot true airspeed, the tailwind represented a hefty 50 percent net increase in speed across the ground. To compensate for the strong tailwind, Universal's computer planned for descent to commence 119 miles out — a 44-mile adjustment. In an identical scenario programmed with zero wind, the computer showed the descent starting 81 miles out. This was reasonably in line with the 3 to 1 rule's suggested descent point of 75 miles from the field.

While using a gauge rather than a precisely calculated formula may strike some as a bit too laissez faire for comfort, on average this fuzzy logic works quite well. Because it is a conservative planning technique, it leaves the pilot a little breathing room when the descent plan unexpectedly changes, a regular occurrence in the high-altitude ATC environment. For instance, if center suddenly asks for an earlier crossing restriction, it can usually still be made without much difficulty. It is normally just a matter of adding drag. Most jet aircraft come equipped with in-flight spoilers, or speed brakes, which can be used to increase descent rates considerably without increasing speed. In some aircraft, the combination of idle thrust and extended speed brakes can result in breathtaking descent rates. Boeing 727 pilots, for instance, have been known to insist, with only slight exaggeration, that if they can see a runway out the front window, they can land on it. While the resulting nose-dive descent does not necessarily endear the cockpit crew to passengers, the capability is there if truly needed. This feature is less common on turboprops. The King Air series, for example, does not have speed brakes, but makes up for it by having high flap and gear extension speeds. The King Air C90 has a maximum maneuvering speed of 169 KIAS but an approach flap speed of 178 KIAS. The gear may be extended at 156 KIAS. Faced with the need to expedite descent, a King Air pilot can extend approach flaps and gear at the appropriate speeds. The result is a big increase in descent rate without a corresponding airspeed increase, just as if the aircraft had speed brakes.

Pressurization is what completes the average turbine aircraft's thoroughbred descent characteristics. In most systems, the pilot selects a rate of cabin pressure change, which the pressurization controller then maintains during descent. The rates required are generally quite modest, 500 fpm or less. A jet aircraft with a cabin altitude of 7,500 feet at FL350, for instance, would need a cabin descent rate of 417 fpm during a typical 18-minute descent to a sea level field.

Pressurization allows a pilot the leeway to make rapid descents that would be unthinkable in non-pressurized types. When temporary, very high rates of aircraft descent are required, the pressurization controller can maintain the programmed rate of cabin descent. Eventually, if an excessive descent rate continues, the pressurization controller will no longer be able to maintain the desired rate of descent in the cabin; at this point a large cabin pressure bump will occur. This is known as "catching the cabin," a situation in which the pressurization controller suddenly tries to get back on schedule. When this happens, the cabin descent rate suddenly increases to ear-popping levels. Most of the time, though, expedited descents can be managed without encountering such a cabin pressure bump.

Jet and turboprop pilots learn early that accurate descent planning is not such a difficult feat. With a good rule of thumb to start with, and plenty of aircraft performance waiting in reserve when needed, it's easy for anyone to get down and look good every time.

Vincent Czaplyski, AOPA 690264, holds ATP and CFI certificates. He flies as a Boeing 737 captain for a major U.S. airline.