There’s an expression you will hear a lot at the airlines: “It’s for dispatchability.” In other words, as the expression goes, a plan is moot once the action starts.

Let me explain. The FAA requires airlines to meet certain criteria before a flight can be released, or dispatched. The captain and the dispatcher need to agree on some other items in addition to the standard IFR flight plan items: fuel, weather, alternate(s).

First on the list are any minimum equipment list (MEL) items. These might be as simple as a burned-out light bulb or as complex as a failed nav display.

Second is performance considerations. Everything in the FAR 121 performance world hinges on the loss of an engine. On takeoff, the assumption is that it will fail at V1, which means an abort is no longer an option. On landing, the assumption is that an engine will fail prior to or during an approach, thus necessitating a single-engine go-around. But, go-arounds usually are less restricted by terrain or obstacles, since you’re already off the ground and have the full length of the runway in front of you. That means you can continue to climb for the full length of the runway, whereas a takeoff climb begins somewhere down the runway. You’ll also find that a number of airports have special single-engine procedures developed for an engine failure on takeoff or landing that are also “for dispatchability only,” because they meet certain climb and performance requirements. In the real world, pilots can (and should) use their best judgment (such as in bad weather).

To further add to the confusion, you may find that at certain airports, the single-engine procedures are only used by some fleets…and among the same fleet type, there may be variations from one carrier to the next on those procedures because of engine differences.

All takeoff data is predicated on losing the most critical engine and reaching the four segments of a climb (beyond the scope of this post). Remember, that’s a worst case scenario. When you hear the “that’s for dispatchability” comment regarding takeoff, it means that once you get to V1 and no engines fail, everything else is gravy. You’ve met all of your regulatory requirements, and nothing else matters. But, you still have to assume the worst, which may mean leaving payload behind.

Another area in which you hear it, and where confusion occurs, affects the MEL. The MEL is designed to give certain relief to the carrier to fly with inoperative components. However, when something breaks in flight, it isn’t necessarily a requirement to begin immediate compliance with the MEL. Here’s an example. Jets and pressurized turboprops have two air conditioning packs that provide pressurization and cabin air. If one is inop, the MEL commonly will restrict flight to 25,000 feet or less to ensure adequate cabin air. However, that requirement is only in effect once the MEL has been used to defer the operating pack for later repair. When the issue comes up in flight, the appropriate checklist will be the guiding document—and it may or may not require a descent to FL250. This can be an important consideration when it comes to fuel and range. As long as the checklist doesn’t require the descent, you can continue to cruise merrily along.

But, once the mechanics defer the pack, you’ll be required to meet any and all MEL requirements as a condition of being dispatched with that particular MEL in use.—Chip Wright