In earlier posts on this blog, I have talked about how airlines plan their flights and how our fuel loads will often limit what we can carry. In this post I want to discuss how performance will affect departure payloads.
The airlines do not plan for a payload capacity just based on what the airplane will carry. That would be too easy. Instead, the most important parameter that must be eliminated first is performance. Two parts factor into this. The first, and simplest, is the requirement to abort the takeoff at V1 minus one knot and still be able to stop on the remaining runway. That’s pretty simple to understand.
The second is an engine failure on takeoff, commonly called a V1 cut or V1 failure. In short, this means that an engine fails at or above the V1 speed, which means that even though the airplane is still on the ground, it will—it must—continue the takeoff. Performance calculations work backwards. Each takeoff consists of a four-segment climb that ends at 1,500 feet agl (there are some slight variations based on certain certification criteria for each aircraft, some of which may be chosen by the manufacturer, and some of which are federally imposed). Each aircraft will have different climb performance capabilities, and will thus reach that 1,500 feet, less one engine, at a different point in time for a given takeoff weight.
Performance calculations, given to the pilots in a useable format by the engineers who did the figuring, essentially work backwards. First, given the weather conditions, especially (but not limited to) temperature, we know, for each airport and each runway, at what point we must reach 1,500 feet following an engine failure. If we are too heavy, we won’t reach 1,500 feet in time, so we must start at a lower weight. As the temperature goes up, the performance goes down, so the maximum takeoff weight goes down. At John F. Kennedy International with a 14,000-foot runway at sea level, it is rarely a problem. At La Guardia, in the same city, with a pair of 7,000-foot runways and a number of potential obstacles not far from the runways, heat becomes a major problem in the summer.
Once we know the maximum takeoff weight based on calculated performance requirements, we can calculate maximum payload. Fuel is the first concern—you aren’t going anywhere without it. Once that is determined, you can determine the passenger and bag count. It gets worse if an alternate is required.
At airports with short runways or at high elevations, I’ve seen days where the CRJ can only carry 30 or 40 passengers. Sometimes the performance penalties are based on the surrounding terrain, and the potential requirement that we be able to fly a single-engine departure in mountainous terrain. Helena, Montana; Roanoke, Virginia; Key West, Florida; Chicago Midway; White Plains, New York; and New Haven, Connecticut, all come to mind as places where I have had to leave a lot of passengers or bags behind–not because of the ability to use two engines to get airborne, but because I need to know that the same airplane will fly safely with one engine shut down.
Winter operations can be problematic as well, because using engine bleed air for deicing operations can degrade performance. So can contaminated runways. Construction projects often wreak havoc as well.
This is a greatly simplified explanation of what goes into performance considerations, but it touches on the highlights. It’s something to keep in mind if you are told by the gate agent or crew that “performance” or “weight and balance” is going to require that you (or your bags) take a later flight.
As inconvenient as it may be, take some consolation that safety is not going to be compromised for the potential of a few bucks for the airline. They’d rather lose your money than your life or their equipment.–Chip Wright