Understanding Takeoff Calculations

Safe Initial Climbs Can Mean Tradeoffs

A pilot’s first takeoff in a jet airplane is a true rite of passage, almost as eye opening as a first solo. Push those power levers forward, and a pilot’s body is soon pressed back against the seat. That feedback is tough to duplicate in a piston airplane. But jet power, speed, and flight safety—not to mention strict certification standards—demand a real understanding of the performance calculations that many new pilots today are trusting to an iPad app.

Ponder one departure myth, for instance—the one that claims jet pilots needn’t worry about engine failures at takeoff because of all the extra power waiting in those big turbines. Not necessarily. Matching true jet performance against these myths becomes much clearer through the scenario-based evaluations now demanded in the new airman certification standards.

The Engine-Out Performance Myth

Consider an engine failure just after rotation. Certainly a Hawker 800XP with one engine idled will climb better than a Cessna 310 with one throttle pulled back, won’t it? But like a twin Cessna, the climb performance of a jet drops off precipitously when one engine stops operating, depending upon how the change is measured.

On a standard day, even at maximum takeoff weight of 28,000 pounds, our Hawker 800XP’s power will rocket the aircraft from sea level to FL370 in about 20 minutes, at about 1,850 feet per minute. If an engine quits at that same weight, however, the airplane’s single-engine climb rate drops to approximately 470 feet per minute—a decrease of about 75 percent. How well the aircraft climbs after that, as in all aircraft, is also dependent upon the skill level of the person behind the controls. Looking back at the nearly negative single-engine climb rate of a light twin such as that Cessna 310, a 500-fpm climb rate might sound impressive. But there’s more to creating a safe departure in a jet.

Imagine our 800XP is departing Aspen, Colorado, an airport nestled between monumental peaks of the Rockies at a 7,838-foot elevation. Today’s outside air temperature (OAT) is 15 degrees Celsius. A quick check of the approach and departure plates highlight the first departure concern: the rocks. The minimum safe altitude (MSA) northwest of the field reaches just above 13,000 feet. Southeast of the airport, the safe altitude climbs to nearly 16,000 feet within just a few miles. Usable runway is 8,006 feet. So no big deal, right?

A close look at the Aspen Six departure is next to see if the Hawker’s takeoff performance can best any published restrictions. Plenty of runway, of course, and a check of the performance section of the aircraft flight manual (AFM) confirms that the Hawker can safely clear the runway—with one small glitch. The aircraft can’t do it at maximum takeoff weight. In a jet, the term usable runway length really needs a note that says “usable sometimes.”

Flying the Segments

Digging a bit deeper into Part 25 certification standards shows that the entire takeoff path, from the runway to 1,500 feet, is divided into four separate segments. Each of them proves exactly what the aircraft can actually do, while also remaining clear of all obstacles. There’s no guessing in jets.

With both engines operating at maximum power, the first segment begins at brake release and liftoff as the aircraft climbs, with the gear in transit, to an altitude of 35 feet above the ground at the V2 airspeed, also known as takeoff safety speed. The reasoning behind a 35-foot measurement point is not relevant to this discussion, except to highlight the point that all Part 25 Transport aircraft calculate first-segment performance to the same point in space—whether for a Hawker 800XP or Boeing 787.

The second segment, with its own performance requirements, begins at 35 feet and continues until the aircraft has reached 400 feet above the ground; normally it lasts only a few seconds when both engines are putting out takeoff power. During a normal climb, the aircraft should be expected to maintain a climb gradient of 2.4 percent.

The third takeoff segment begins at 400 feet as the pilot reduces angle of attack, retracts the flaps, and allows the aircraft to accelerate, headed for the fourth and final segment. The final segment climb gradient for a twin-engine jet drops here to just 1.2 percent.

Most departure procedures near restricted airports speak to aircraft performance in climb gradients, a percentage or an altitude per nautical mile. All are calculated assuming both engines are operating at full power. The Aspen Six chart in our example, however, says ATC could require a climb rate of 840 feet per nautical mile all the way to 16,000 feet, a climb gradient of nearly 8 percent—or about a 2,000-fpm climb. But if an engine flames out on takeoff, we calculated earlier that 500 fpm is all we’ll probably see with one set to maximum power. If you’re already in the clouds at this point, it means a collision with the granite is inevitable—hence this extra pretakeoff planning. Of course, the pilots could just cross their fingers, hoping both engines will work, at least until the airplane has cleared the rocks. To ensure the safety of the passengers, crew, and the aircraft in this example, however, the pilot in command needs to offload fuel and/or payload until the performance numbers confirm the aircraft can clear the granite if an engine quits.

Most experienced pilots realize the need to comply with these performance requirements. Still, there are some pilots who either don’t know the numbers or don’t care. For them, the big question when flying out of places such as Aspen is, “Do you feel lucky today?”

Rob Mark

This story originally ran in AOPA Pilot’s, Turbine edition. Reprinted with permission.