Proficient Pilot: Point of no return

The failure of the Pratt & Whitney PW4000 turbofan engine on this Boeing 777—the fifth one built—appears to have been caused by the fatigue fracture and separation of a compressor blade.

The crippled flight returned to Denver and landed safely. The passengers were then transferred to another triple seven. Coincidentally, this was the fourth one built. But here is the bizarre part. The replacement ship had experienced a similar event on a flight from San Francisco to Honolulu on February 13, 2018. Operating as United Flight 1175, the number two engine failed catastrophically as the flight began its descent toward Honolulu. There was a loud bang and a violent shaking of the airframe, the result of a compressor blade failure and separation. Like its sister ship in Denver, the flight landed safely.

There were so many similarities between these two events that the NTSB could almost cut and paste the accident report from Flight 1175 onto its report for Flight 328.

The successful outcomes of these similar events can be attributed—at least in part—to the good fortune that each catastrophic engine failure occurred in proximity to an airport. But what if one of these failures had instead occurred halfway between California and Hawaii? What if the violent shaking of the failed engine would cause it to shift position in its mount, resulting in a dramatic increase in drag? What if this added drag made it difficult—if not impossible—for the lone remaining engine to maintain altitude?

The navigator announced dramatically, “I have news for you guys. We just passed the point of no return.”In other words, this scenario could have occurred anywhere along the oceanic portion of the route, which confirms anew that fate really is the hunter. The suggestion of an engine shifting in its mount is reasonable to those who saw the Flight 328 video on the nightly news of a shaking right engine recorded by a passenger from a cabin window aboard the Denver event.

Such a mid-Pacific emergency would be reminiscent of the situation dramatized in the 1954 grandaddy of all airline disaster movies, The High and the Mighty.

The first dramatic moment of the film occurred when a brief, inexplicable airframe shudder caught the crew’s attention. Not long after that, the navigator announced dramatically, “I have news for you guys. We just passed the point of no return.” The problem with that proclamation is that passing the PNR had no meaning. The PNR is that point beyond which a flight cannot return to its departure point. But so what? When an airline flight is at the PNR, it is typically closer—usually much closer—to its destination than its departure point, so why would its pilot want to reverse course (unless the weather ahead made continuing unwise)?

More important and practical than the point of no return is the equal time point or ETP. This is the point along a lengthy oceanic leg at which it would take just as long to continue to the destination airport as it would to return to the departure airport. In other words, when at the ETP, it would take the same amount of time to continue the flight as it would to return.

There are different kinds of ETPs that can be calculated prior to the departure of an oceanic flight in a jetliner (or bizjet). One is useful in case of an engine failure. In other words, if an engine were to fail en route from San Francisco to Honolulu prior to reaching the ETP, the flight would return to California; if the engine were to fail after passing the ETP, the flight would continue to Hawaii. The ETP calculation takes into consideration the need for a twin-engine jetliner to drift down to and cruise at some lower altitude—such as 25,000 feet, for example—because it will be incapable of maintaining high-altitude cruise on one engine. The ETP is predicated, of course, on the new single-engine cruise airspeed and the winds aloft at the lower altitude.

Another ETP might be calculated for use following loss of cabin pressure. At such a time, the crew of a jetliner would execute a rapid descent to a typical altitude of 10,000 feet where those aboard can breathe without supplemental oxygen. This ETP, of course, is predicated on the adjusted cruise airspeed and winds aloft at 10,000 feet.

The point of no return has no practical meaning in civil aviation—unless the pilot does something dangerously foolish and from which there will be no return.

barryschiff.com