March 1, 2010
Robert P. Mark
Think back to the excitement of your first takeoff. Remember the thrill as you pushed the throttle to the panel and watched the tachometer wind up? The airplane began to rumble and shake, gathering speed until finally it leapt into the sky. All your instructor asked of you was to steer the airplane down the middle of the runway by making your feet dance on the rudder pedals. Takeoff planning involved little more than knowing when the instructor would tell you to pull back on the control yoke.
Later, as your flight training progressed, takeoff planning became more complex as density altitude, useful runway length, and weather found their way into the go/no-go decision process. New pilots must learn to identify when the takeoff is or is not progressing properly, and—in either case—know what to do. The decisions are much the same flying turboprops and jets, just a little more complex.
The thinking behind how large aircraft can and will perform, and the necessary mitigation of risks surrounding their complexity, is wound into the way they’re manufactured. Small training airplanes and some twins normally are built under FAR Part 23, while Transport category aircraft are constructed to the more demanding standards of FAR Part 25. In any large airplane, for example, there’s certainly a concern about available runway for takeoff, as in any other aircraft—but in twin-engine airplanes there is also a concern about how the airplane will perform after it leaves the ground if one engine stops running—and even whether there is enough time and pavement to stop the aircraft halfway through the takeoff roll if things don’t feel right.
Part 25 turbine twins must demonstrate measurable takeoff and climb performance if an engine stops at a variety of points in the takeoff roll. Under Part 23, a light twin is not required to demonstrate any climb performance after an engine quits. Another part of the large-aircraft calculation process means determining obstacle clearance related to any changes in takeoff or climb performance. Calculating all of the possibilities, in fact, is what makes takeoff planning more time-consuming in a turbine airplane.
And sometimes the unexpected occurs on takeoff. In September 2008, a Learjet 60 crashed on takeoff at Columbia, South Carolina. Witnesses said the takeoff on the 8,600-foot runway appeared normal until sparks were observed beneath the aircraft. Early reports surmise that one or more of the tires came apart, no small problem when traveling at high speed. At a speed of 136 knots, possibly in excess of the calculated takeoff rejection speed, the captain attempted to abort the takeoff.
Unable to stop on the remaining runway, the Learjet sailed off the end, through some approach lights, and crashed into an embankment. Four of the six people on board, including the two pilots, were killed. The remaining two passengers received serious injuries. Statistics show that high-speed takeoff aborts seldom have a positive outcome. That’s why most pilots prefer getting the airplane airborne to deal with most emergencies.
Good takeoff planning begins with a clear understanding of the challenges ahead, the load in passengers and baggage, as well as the fuel needed to take them to their destination with reserves. The Hawker 800A, for example, weighs in at 27,400 lbs. maximum takeoff weight. The aircraft is capable of carrying about 10,000 pounds of fuel and about 1,000 pounds of people and bags. Assume the airplane will carry a crew of two, plus four passengers, on a 1,800-nm trip—which means filling the aircraft with fuel. That should bring the aircraft weight up near maximum.
The weather at the departure airport is marginal VFR in light rain, with a temperature of 74 degrees Fahrenheit. Available runway is 6,900 feet. There are hills that sit about 1,500 feet above field elevation located three nm from the airport, about 1,000 feet to the left of the extended runway centerline.
What the crew wants to know is how much runway they’ll need to get airborne, how well the aircraft will climb with both engines operating, and what performance to expect if one of the engines dies at a critical point in the takeoff roll. They’ll also want to know for certain—based upon the wind and the temperature—at what point in the takeoff roll it is safer to continue the takeoff if one engine rolls back rather than attempt to stop on the remaining runway. The advantage of a Part 25 turbine twin is that it’s designed to complete the takeoff and climb on one engine to a safe altitude, but the crew must perform the calculations correctly. Factors such as runway slope, pavement condition, and the possible presence of snow or slush on the runway must also be accounted for during the calculation process.
Whether takeoff numbers are determined using the tables located in the aircraft flight manual or a computer, the results the pilots see are the same—a combination of indicated airspeeds and distances for reference. The first data line is the Balanced Field Length (BFL). This indicates how much runway the aircraft requires to accelerate to its first reference speed, called V1 or takeoff decision speed; lose an engine; recognize the problem; and stop the aircraft on the remaining runway. Thrust reversers are not factored into the distance calculations. Once the aircraft accelerates past V1, attempting to stop is pointless since we’ve already calculated there is not enough runway left to halt the aircraft. The aircraft must take off.
If the preflight data show that more runway is required than the 6,900 feet available, the decision is easy: Remove some weight or wait for the temperature to drop. Less weight, of course, could translate into less fuel, which might now mean an en route fuel stop when a nonstop was initially planned.
The next speed encountered on takeoff is VR or aircraft rotation speed. Here the pilot flying takes his hand off the thrust levers—the aircraft is committed to the takeoff so the levers are now only a distraction—and hauls back on the yoke or sidestick to raise the nose of the aircraft for takeoff. On a two-engine takeoff, big airplanes behave like rockets. With one sick engine, performance is significantly reduced, as are the pilot’s options. With one engine producing little or no power, however, many thrust systems will automatically boost thrust on the good engine to help the aircraft accelerate to V2 (single-engine safety speed) and the magic 35-foot altitude the aircraft is expected to achieve when crossing the departure end of the runway as the gear is retracted. This portion of the takeoff profile, outlined in Part 25, is called the first segment.
Once the gear is up, a single-engine departure transitions to the second segment, a phase where the crew does its best to attain best angle of climb speed and essentially put as much distance as possible between the aircraft and the ground. The flaps remain in the takeoff setting for this portion. This is the segment when the cockpit becomes very quiet since there’s little to do except wait for the aircraft to climb normally to about 1,500 feet agl.
The second segment is the most critical part of the takeoff now that the aircraft is actually flying. That’s because although the manufacturer promises the airplane will perform to certain standards, it cannot guarantee the aircraft will perform well enough to clear any obstacles, such as mountains, near the departure airport. Our Hawker departing Aspen, for instance—where the field elevation alone is more than 7,000 feet (think about that density altitude on an 80-degree F day) —may well perform just fine when both engines are turning. But should one quit after V1, there might not be enough performance to make the second segment climb gradient necessary to clear the mountains. If the weather is good VFR, some crews might elect to take off knowing full well they cannot make second segment if one quits. Their plan? Remain VFR and steer around the hills. I’ve tried it in the simulator and it’s, well, exciting.
In the third segment, once the aircraft reaches 1,500 feet, the flaps are retracted and the aircraft is allowed to accelerate. At this point, the flight proceeds almost normally, as the crew either attempts to restart the dead engine or plans the next step, a single-engine approach back to the airport they departed from—unless, of course, it’s Aspen. In that case, most pilots would head for Denver International with long runways and no mountains. Take a scenario such as this, throw in a healthy dose of density altitude, and then go crunch the numbers. At heavy weights, chances are you’ll discover that meeting balanced-field requirements probably will be impossible in the first place.
In order to meet the runway requirements under high density altitudes, your mission planning will probably take a beating. You may have to trade fuel for passengers in order to be light enough to ensure safe runway performance, and that means less range. Now you know why manufacturers use Aspen to tout their airplanes’ ranges after taking off from hot-and-high airports.
Robert Mark won the Airbus Aerospace Journalist of the Year award in 2009.
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