Balancing act

“Lessons from a Performance Chart” focused on takeoff and climb performance, but the general principles apply to many other operating handbook charts. One participant, Mike, emailed later to say he had heard of the balanced field concept for takeoff used by commercial operators but confessed he didn’t understand it. Mike asked if I could offer an explanation and wondered if there were any takeaways that general aviation pilots might extract for safer operations.

Having never flown for a commercial operator, I found myself a bit flat-footed on the concept, so I dived into various flight manuals to brush up on the topic. What I found was honestly confusing, especially in the FAA’s cumbersome and incomplete explanations. It’s easy to become mired in the definitions that make the reader hop back and forth among the various sections of the Code of Federal Regulations (CFRs) and miss the spirit of the balanced field concept. The following discussion is not meant to detail the definitions and requirements in the CFRs. Rather, I hope that, armed with a general understanding, you will find the FAA material more accessible.

Let’s start by assuming that a multiengine airplane departs a sea-level airport on a standard day with no winds. The airplane begins the takeoff roll and one engine stops producing thrust when the aircraft has achieved VEF airspeed (the speed at which the critical engine is assumed to fail during takeoff). From there, the aircraft accelerates to a reference airspeed, V* (more on this later). The pilot then commands idle thrust on the remaining engines, applies maximum braking, and brings the aircraft to a stop. The accelerate-stop distance is the total distance traveled by the airplane. For a fixed V*, a lower value of VEF results in an increased accelerate-stop distance. For a given VEF, increasing the reference speed V* has the same effect.

If the aircraft has sufficient power remaining, a pilot might achieve a better outcome by continuing the takeoff in the above engine-failure scenario. After engine failure at VEF, the airplane continues to accelerate to the rotation speed VR. From there, it continues to climb until it reaches takeoff safety speed, V2 (a speed that provides a reasonable climb rate and a buffer over stall speed), at a height of 35 feet above the ground. The total distance becomes the accelerate-go distance.

If an engine failure occurs at a low airspeed, it makes sense for the pilot to throttle back and bring the airplane to a halt. At higher airspeeds, though, the pilots should continue the takeoff and handle the failure as an in-flight problem for the ability to stop without impacting obstacles is compromised or impossible. There is a middle range of airspeeds where either, or perhaps neither, action will produce a safe outcome. It all boils down to the runway available.

The attached figure shows that a sufficiently long runway provides a range of airspeeds for which either decision—stop or continue the takeoff—should have a safe outcome. The changeover airspeed (the above reference airspeed V*), called the takeoff decision speed or V1, is selected to be somewhere in the middle of that range. Such a long runway, called a balanced field, offers a greater margin of safety. On the other hand, if the runway is too short, there is an airspeed band during which neither decision will end in success. A barely balanced field is a runway that provides just one value for V1 and no wiggle room for a pilot who fails to act correctly and quickly upon engine failure that occurs around V1.

Commercial operators typically require their pilots to operate from balanced fields.

Note that the above assumes that the aircraft has sufficient thrust to be able to meet various climb gradients with an inoperative engine. Multiengine trainers often feature anemic climb performance, so it may be wiser to land on the remaining runway even if the engine failure occurs after lift-off and landing will mean going off the end of the runway. And those of us who fly single-engine aircraft have no choice but to work with the runway, and what lies beyond it, if the engine fails during the takeoff roll.

The main takeaway for general aviation pilots is that commercial pilots who use the balanced field concept don’t cut it close. They allow themselves a Plan B and a safety buffer to allow for the less than perfect actions of a pilot faced with an unpleasant surprise. With the heat of summer upon us, it’s good to remember that no takeoff is mandatory. Ensuring we have a well-balanced runway, or at least one long enough to provide options on a bad day, is something we owe to our passengers and ourselves.

Catherine Cavagnaro teaches aerobatics at UOS and is the Gaston Swindell Bruton Professor of Mathematics at Sewanee: The University of the South.