Taking off

Business Jet Traveler » August 2007
It is your responsibility as a passenger to ensure that the preflight safety
Wednesday, August 1, 2007 - 5:00am

All takeoffs are optional but all landings are mandatory.



It's one of those clever dark-humor catchphrases that permeate aviation and it speaks an unerring truth. Once you break ground and clear the security of the runway, you've left behind the one option that absolutely guarantees you will not have an airplane crash today.



While the level of commitment in taking off is undeniable, actually pushing throttles forward and keeping the nose straight until you're airborne usually involves a relatively low level of the pilot's skill. In fact, manyinstructors allow students to perform the takeoff on their first lesson. As the airplane accelerates, the wings begin to develop lift and the controls come alive as the airflow over them increases-imagine the rudder of a boat as it picks up speed.

In a two-pilot jet, there is a "pilot-not-flying" to call out when sufficient speed shows on the gauges. But the "pilot-flying" already feels the airplane wanting to lift off and climb toward its natural habitat. It is about to abandon its role as a clumsy land vehicle and assume its rightful mandate as a flying machine. This truly is a magical moment.



It's what comes before the moment of liftoff that is the really complicated part. There is a lot of preparation that goes into safely taking off, whether it's for a weekend sightseeing flight in a four-seat propeller airplane or a globe-girdling business jet trip to third-world countries.



Takeoff planning starts with a comprehensive evaluation of the three-way matrix of runway length; fuel required for the trip; and the weight of the passengers and their luggage. All takeoff planning centers on weight, and knowing how much the required fuel will weigh, added to the weight of the passengers and cargo, is the starting point. For every specific airplane, there are safe margins that must not be compromised, even though there is a substantial built-in buffer zone.



In the case of a multi-engine jet, the crew must determine mathematically that the airplane will be able to accelerate to its precalculated takeoff speed (the heavier the weight, the higher the required speed for liftoff) and then be able to stop in the remaining runway should an engine fail before that speed is reached. Once the aircraft has accelerated beyond the predetermined number of knots, the pilot knows it is safe to continue the takeoff even after an engine failure.



Let's say the pilot knows that it will take X pounds of fuel (including ample reserves) to fly from departure point to destination. (Takeoff calculations are why jet pilots calculate fuel load in pounds. A gallon of jet fuel weighs about 6.7 pounds.) The passenger/cargo load equals Y pounds. The runway length is Z feet. Maybe the total weight amounts to too much for the airplane to carry safely. Either someone (or something) has to stay home, or they must take off with less than full tanks and stop along the way to refuel. It's the crew's responsibility to keep X, Y and Z within the airplane's performance envelope.



Besides the calculations of how much weight is on board for takeoff, there is also the question of how the weight is distributed. In most cases, the margins are sufficient that the crew knows there is little concern about where everyone is seated in the cabin. But don't be surprised (or offended) if a full passenger load with ample baggage elicits a request from the pilots for the heavier passengers to sit nearer the front-or back-of the cabin. Different airplanes have differing balance dynamics and the crew is trained to know where weight must be distributed to keep performance within specs.



When evaluating runway requirements and weight calculations, other factors come into play as well, including temperature and humidity-both of which affect the ability of the engines to develop their maximum power and the wings' ability to generate lift. The hotter and muggier the conditions, the more runway is needed. And if the airport is at a high elevation above sea level, the thinner air also enters into the planning. In winter, high temperatures aren't a worry, but a slushy runway adversely affects takeoff performance and must be figured into the equation as well.



Wind speed and direction are also important variables. A pilot likes nothing better than pointing the nose directly into a strong, steady breeze right down the runway. Airplanes usually take off into the wind, to maximize the speed of the air over the wings, while the speed over the ground remains as low as possible. That gives the effect of already having 20, 30 or more knots of airspeed before the takeoff roll even begins. That can amount to a real performance bonus if the runway is a tight fit.



One caveat that pilots learn-especially in the U.S. Midwest-is to beware of gusty winds during takeoff. Even if the wind is directly down the runway, the airplane can be "fooled" if the wind speed varies significantly. It can be embarrassing (or worse) to lift off into a 30-knot wind and suddenly lose 20 knots in the middle of the takeoff. Losing that much wind all of a sudden can be the difference between safe flying speed and stall speed, causing the airplane to sink back onto the runway with a thud. For this reason, pilots are trained to add extra speed to the takeoff calculations if gusty conditions prevail.



Not infrequently, the wind blows perpendicular to the runway. It's called a crosswind, and some motorists are familiar with its effects, especially truckers and drivers of large RVs. Pilots must consider crosswinds in the landing phase, but takeoffs are also important when it comes to setting up the controls to cope with the effects. Pilots anticipate the wind direction and speed based on known conditions, visually check the windsock for the ultimate in real-time information and position the aircraft controls accordingly as they add power at the start of the takeoff roll. A strong wind can slam into the tall tail of the airplane, causing it to try to "weathervane" on the runway. Once the airplane breaks free from terra firma, it is immediately subject to the full influence of the wind. So pilots make sure the proper control corrections are already in place before lifting off.



Finally, it is your responsibility as a passenger to ensure that the preflight safety briefing isn't taken lightly. It's remarkable to me how few people pay attention to these briefings. I've had intelligent, educated people ask me why I bother to have seat belts in my airplane, since any crash is bound to be fatal. Consider that an accident in the takeoff phase-as unlikely as it is-could easily occur at highway traffic speeds or lower. Seat belts properly fastened are the best hedge against injury in an aborted takeoff. And remaining conscious and uninjured is the best way to ensure that you can quickly and safely evacuate the aircraft. The message is simple and compelling. Listen to the safety briefing. And buckle up.

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