By Vincent Czaplyski (From AOPA Pilot)
Some years ago a pilot acquaintance of mine described how, during takeoff in a twinjet, a tire blew out. Pieces were swallowed by an engine, which then failed shortly after the flight reached V 1 (see definition below). He wisely elected to continue the takeoff, then returned for landing at the departure field. I congratulated him on the successful handling of the emergency a scenario twinjet pilots train for their entire careers, but with rare exception will never experience. But there was more, he told me. Exactly a week later, another engine failed during takeoff, this one caused by the ingestion of a large bird. As before, the crew continued the takeoff and immediately returned for landing.
Whether he looked for a desk job in the chief pilot's office afterward, I don't know. His chances of winning Powerball are probably still better than the odds of experiencing two such engine failures. But while lottery windfalls are all about luck, twinjet takeoff performance on one or both engines is about design criteria and careful planning. By staying within the aircraft's certificated dotted lines, operators can take the performance guesswork out of takeoffs normal or otherwise.
FAR Part 25 details the design performance expectations of Transport category jet aircraft in all phases of flight. Takeoffs are arguably the most critical phase. Understanding a few of the terms and concepts found in Part 25 is useful in understanding how safety margins for twinjet takeoffs are derived.
V MCG is not a speed pilots dwell upon during most twinjet takeoffs, since it is usually reached well before V 1 speed. If an engine fails prior to V MCG, the correct response is to reduce power on the remaining engine and reject the takeoff. Since speed is below V 1 anyway, the decision is clear. A problem occurs in those rare cases when calculated V 1 is less than V MCG. By definition, a takeoff should normally be continued if an engine failure occurs after V 1. But aircraft control could be lost if takeoff was continued on one engine while still below V MCG. What to do in this Catch-22 situation? Whenever calculated V 1 is less than V MCG, FAR Part 25 dictates that V 1 be increased so that it is greater than V MCG.
Within the constraints of balanced field length requirements and various performance speeds, every jet takeoff must also satisfy the most conservative of three performance weight limits. By observing the following limiting weights, an operator is assured that the aircraft will perform according to the design expectations of FAR Part 25.
The structural limit weight is the aircraft's certified maximum takeoff weight.
The runway limit weight is the maximum weight for existing takeoff conditions that still allows the aircraft to meet balanced field length requirements on a particular runway. Factors influencing this value include wind direction and speed, ambient temperature, field elevation, runway slope, and, of course, length.
The climb limit weight relates to that portion of the takeoff profile from 35 feet above the takeoff surface until the aircraft has climbed to 1,500 feet above it. Aircraft weight, altitude, and ambient temperature are the variables that most influence climb ability. FAR Part 25 requires that a jet be able to clear all obstacles in the takeoff flight path when continuing takeoff with a failed engine. Obstacle clearance is based on twinjets maintaining a minimum 1.2-percent single-engine climb gradient when between 400 and 1,500 feet above the takeoff surface. (Note that this is only half of the single-engine climb gradient required in the climb to 400 feet.) Once reaching 1,500 feet, and in the en route configuration, all climb limit planning considerations are thought to be satisfied.
If the runway length is the limiting factor, selecting a higher takeoff flap setting will sometimes shorten balanced field length requirements. A higher flap setting typically results in lower V 1 and V R speeds, meaning that less physical runway length is required for the takeoff. Payload can be increased if desired, until the aircraft weight again rubs up against the runway limit for the new flap setting.
But what if the climb weight limit is reached first during takeoff planning? In other words, the physical length of the runway is not the limiting factor. Rather, obstacle clearance criteria along the takeoff path determines the limiting weight. In this case, the extra runway length can be used to permit a higher rotation speed, and a higher takeoff weight. Attaining a faster speed on the runway means the aircraft's energy state is greater once it lifts off. This energy can be used to climb faster, thus better satisfying climb requirements. The takeoff roll will be longer, but balanced field length requirements can still be met.
Another useful technique for increasing allowable takeoff weight is to make a bleeds-off takeoff, if this is an option in the particular aircraft. Engine bleed air is normally taken from the engine compressor section and used to power air conditioning, anti-ice, and other systems at the expense of a slight decrease in available engine thrust. By using this air instead for additional thrust, the aircraft's takeoff performance is enhanced.
Most takeoffs are perfectly normal, and most twinjet pilots will never lose an engine during takeoff. But it's reassuring to know that in either case, a successful outcome is all part of the plan.