Precision Descent Planning for Palm PDA and Windows

About desPlan

Features

Ideal for MSFS pilots and VATSIM FSS
Advanced optimization algorithim
Precise descent profiles
Automatic TAS adjustments for altitude
Quickly calculate descent rates
Standard descent rates available
Easy to use for on-the-go clearance delivery
Optimal descent paths for crossing restrictions
Updated in 2007 for Palm PDA
Windows version available

Description

Using desPlan a pilot can eliminate the headache of finding a descent rate to meet a crossing restriction, or other target altitude. Sure, there are rule of thumb - we know: "Divide groundspeed by 2 then add a zero; "Multiply ground speed times five"; "add a zero to indicated air speed, then divide by two"; and so on. Which rule does one choose? Conditions are always different. Planning a descent profile to achieve a waypoint crossing at a specific altitude requires more than simple rules of thumb, especially when the change in altitude is several thousand feet or more.

Mathematically, it looks simple: subtract the target altitude from the current altitude and divide by the estimated time of arrival at the waypoint - the result is a descent profile. However, this procedure may require constant adjustments of speed, and recalculations of descent rates along the descent path.

To see why this is, consider an aircraft cruising 250 knots indicated airspeed (IAS) at FL450 (i.e., 45,000 feet). Air traffic control (ATC) directs the pilot to descend and cross GARZO, a waypoint 75 nautical miles distant, at 12,000 feet. Using the procedure described above, the pilot determines that he must descend 33,000 feet (45000-12000), and that at 250 knots it will take about 18 minutes. Dividing 33000 by 18, he calculates a descent rate of 1833 feet per minute. A little more than 11 minutes later he crosses the waypoint, and discovers (to his horror) that descending through FL240 he has busted his crossing restriction.

What happened? Our hapless pilot might think it was a ferocious headwind that placed him in such a predicament, not so. What really happened was he neglected to consider air density, which decreases with altitude, and makes planes more "slippery" and therefore travel faster. At FL450, when the indicated airspeed was 250 knots, the true air speed was 475 knots! Slippery indeed.

Suppose our pilot had the foresight to account for air density, and instead of using indicated airspeed, used true airspeed to determine the descent rate. He would calculate a descent rate of 3483 feet per minute - this would put his flying machine at 12,000 feet and about 14 miles from the fix. Acceptable, yet not very precise - the goal is to cross the waypoint at 12,000, exactly. Our pilot reached the target altitude far too soon, and as a result burned more fuel than necessary.

One possible solution might be to average the descent rates ((1833+3483)/2) to get 2658 feet per minute. This is a little better than the previous two scenarios, but is still lacking. Even then, our pilot would would reach the fix some 4000 feet above the crossing restriction.

What to do? Suppose the descent rate could be adjusted continuously along the flight path. At the outset, the descent rate would be 3483 feet per minute as previously described. Some 11 minutes later the rate of descent would be 2273 feet per minute - the rate at which the descent profile changes along the flight path decreases, and not only that, it decreases at a decreasing rate - that is, early on in the descent profile the descent rate would have to be decreased about 2.13 feet per minute each second - while later, the rate of change (of the descent rate) would be a mere 1.89 feet per minute each second. Had the descent rate been adjusted continuously as just described, the aircraft would have crossed the waypoint at exactly 12,000 - truly a precision descent, yet it would be impractical for a pilot to calculate true airspeed along every point of the descent path, and adjust the descent rate every second.

Is there a single precision descent rate that could be used along the entire descent path? -- YES

Intrducing desPlan: a new planning tool to quickly and accurately compute an optimal descent profile; a single rate of descent that simultaneously gets you to the waypoint, and at a target altitude.