Every summer, it seems, there are days where the temperatures somewhere are hot enough that the media has reports that airplanes can’t take off. It is easy to scratch your head and ask how it is that an airliner can’t depart, even in a high density altitude environment. The most common place for this seems to be Phoenix.
Two things can drive this. The first is pretty simple: Hotter temps mean higher ground speeds for takeoff, and those speeds can mean that the speed limits for tires can be exceeded. Knowingly exceeding a limitation is never acceptable, and the result is usually a cancelled takeoff, or more likely, a reduced payload to reduce the speeds.
The second issue is performance once airborne. But it isn’t the all-engine performance that is the issue. It is single-engine performance, and more specifically, it is the single-engine performance that would be required when losing an engine at the worst possible time, which is right at the speed known as V1.
V1 is known as the takeoff decision speed, but more accurately, when the speed reaches V1, the crew is committed to taking off, with very rare exceptions (I know of one crew that aborted after V1 because the elevator was jammed). The FAA requires that manufacturers of FAR Part 25 certified airplanes be able to demonstrate that a takeoff can be safely continued after losing an engine at V1. They further define the climb segment as being four distinct segments, all of which have certain requirements: liftoff to 35 feet; 35 feet to 400 feet; an acceleration segment; and 400 feet to 1,500 hundred feet.
Further, all of this must be done while meeting certain climb gradient criteria without violating any of the TERPS parameters. One of the challenges comes with what can best be described as “non-standard” climbs. These can (and often are) be driven by obstacles or terrain in the departure path. This is especially true if an airport has been shoe-horned in or if the area around the field has been developed in such a way that it is no longer in compliance with FAA criteria.
When you learn to compute airline performance data, you aren’t all that concerned with all-engine performance. You are instead concerned with how to meet each of the four segments of climb. You may not know exactly where the TERPS concern is, but you know that something in the departure path is an issue, or that the runway is too short to accommodate the necessary acceleration after losing an engine at V1.
In my next post, I will discuss the work-around for some of these challenges, known as special engine out procedures. These procedures are essentially an alternate method of compliance that allow for the maximum possible payload (and revenue) without compromising safety. You don’t need to be Chuck Yeager to fly these safely, but you do need to thoroughly review and brief what the steps are, and be prepared for the unlikely to become your new reality.—Chip Wright