Book Review: Aircraft Performance Myths & Methods

 - January 27, 2010, 10:51 AM

Jim Deuvall has written and published a handy primer on aircraft performance that can help pilots expand their understanding of how jets perform. In 10 concise and easy-to-digest chapters, Aircraft Performance Myths & Methods explores performance myths, which Deuvall characterizes as “unsubstantiated, commonly held belief[s] that ha[ve] come down through the ages as truth.” Deuvall is a pilot, former simulator instructor and current owner of CAVU Companies, which published the book and produces aircraft performance software.

The performance myths revolve around the most important areas of operation– takeoff, climb and landing–and each chapter addresses specific issues that pilots tend to generalize into rules of thumb that, Deuvall writes, “are largely at odds with approved aerodynamic principles, regulation or practices.” The myths are:

• The AFM performance manual section is an exercise in certification and contains only dubious operational data, which has little effect on daily decision-making.
• Second segment is always the most limiting.
• The best option, upon losing an engine, is to maintain V2 until clear of obstacles.
• The FMS provides all the takeoff gradient information necessary.
• A range of misperceptions, including, “Safety is improved if V1 is lowered” and “if you lose an engine above V2, pitch to V2.”

In exploring these myths, Deuvall delves into details of aircraft performance that should be second nature to all jet pilots, but often are not. He explains the reasoning behind the regulations and the physics behind the reality that underlies each myth.

Runway Limits

A good example is the discussion of runway limits, which opens with a detailed explanation of V1 speed. Many pilots believe and are taught that V1 is when pilots need to make a decision, hence the phrase “decision speed.” But, he writes, “V1 speed is actually the very end of the decision process, the point when the first action, as a result of having made a decision previously, must take place. So, if the decision is not to continue the takeoff, the first action (pull back on the thrust levers or apply brakes) must occur before V1 is reached. The aircraft is actually accelerating up to V1 and decelerating beyond during an abort.”

There is a V-speed that defines “the latest instance an engine can fail,” according to Deuvall, called Vef (engine failure), and “it obviously occurs well before V1.” Even after the engine fails at Vef, the jet continues accelerating and the “momentum of the aircraft is increasing exponentially.”

The pilot can choose either to try to dissipate that momentum using the brakes or deal with the emergency in the air.

Deuvall illustrates the problem using the example of an unidentified jet (possibly the Learjet 60 that overran the runway in Columbia, S.C., during takeoff following tire failure in September 2008). In this case, he writes, “An aircraft crew attempted to abort a takeoff close to V1 after a tire failure. Keep in mind that brakes ‘supply’ the energy which is needed to counteract momentum. Let’s say the tire failure occurred at Vef. The aircraft is accelerating not on one engine, but two, at V1. Is the momentum higher or lower? Clearly the momentum is higher and increasing faster than if one engine had failed. Now, turn to decelerating the aircraft.”

He notes that the greater momentum and exponentially higher energy required to stop the jet is now fighting against only one set of brakes due to the failed tire.
“Here is where the Pavlovian response to abort at V1 without regard for the physics involved can, and often does, lead to disaster.”

Deuvall adds, “In the simulator, pilots will often mistake a tire failure for an engine failure (and vice versa). Little consideration is given to this during the pilot brief.”

Engine Failure Procedure

Another fascinating analysis is Deuvall’s exploration of the question of whether a pilot should pitch to V2 after a post-liftoff engine failure, assuming the airspeed is already faster than V2. AFMs tell pilots not to slow to V2, but Deuvall offers an explanation of the drag curves that helps the reader understand why the higher airspeed is safer, because at V2 more thrust is required to overcome drag and less is available to provide lift.

The example in this case is the American Airlines DC-10 that crashed on May 25, 1979, after a wing-mounted engine broke off during takeoff. “Shortly after, the left engine departed the wing, taking with it the hydraulic pressure that kept the slats on that side of the aircraft extended. With the left outboard slats retracted and all the others extended, the lift of the left wing was reduced and the airspeed at which that wing would stall was increased. The stall speed of the left wing was now six knots above V2. Decelerating to V2, which the crew was trained to perform with an engine failure and the [flight director] command bars indicated, caused the left wing to stall.”

If the pilots of the doomed DC-10 had flown V2+10 “control would have been maintained,” Deuvall writes.

The book is full of similar useful information and many detailed explanations and illustrations, along with tidbits like one at the end that questions the teaching that hydroplaning occurs at nine times the square root of tire pressure. With some modern tires, this is not true, according to Deuvall, and pilots should plan for a more conservative six times the square root of tire pressure.

“I look to you,” Deuvall addresses the reader, “to confront the myths where they may arise.”