Know Your Aircraft’s Specific Handling Characteristics, Part 2
The first part of this article series discussed hours logged doesn't necessarily equate to experience with specific aircraft handling characteristics.
A Learjet 24 was in cruise flight at FL450, enroute to McAllen, Texas, from Casper, Wyoming, when the flight crew made initial contact with Albuquerque ARTCC. About 1 min. later, the flight crew failed to respond to a frequency change instruction and the aircraft's transponder beacon code was lost. The controller made several attempts to contact the aircraft but to no avail. Witnesses at Felt, Oklahoma, heard an aircraft overhead at a very high speed. One witness stated it was in a descent angle of about 45 deg. before it struck the ground. The aircraft was descending at a frightening rate of 10,000 fpm, and NTSB computations computed an average descent angle of 56 deg.
No flight crew would deliberately put an aircraft into such a flight condition, so what could have led to this, and just as importantly, what would be the proper recovery procedure? The “standard” nose-low recovery is to retard the throttles, roll the aircraft upright, deploy the spoilers or speed brakes, and pull the nose up to the horizon. Many will recognize that as the standard recovery technique prescribed in U.S. Air Force Manual 51-37 for an unusual attitude recovery.
Indeed, the flight crew of this Learjet tried doing the same recovery procedure, but unfortunately it did not work. The NTSB concluded that an overspeed condition would have prompted the flight crew to extend the spoilers since that is a natural reaction to an overspeed. Moreover, this procedure was recommended in earlier versions of the aircraft flight manual (AFM). Portions of an outdated copy of a Model 24 AFM were recovered from the wreckage, and its recommendation for recovery from an overspeed condition stated: "If Vmo is exceeded: (1) extend spoilers; (2) reduce thrust to idle; (3) level wings if required; (4) rotate nose up not to exceed 1.5 Gs.”
The maximum operating Mach number of an aircraft can be limited, in part, by its marginal longitudinal stability characteristics. Beyond that speed, the aerodynamic center of pressure begins to shift rearward, inducing a nose-down pitching moment on the aircraft. Unfortunately, the position of the spoilers on the Lear 24’s moderately swept wings creates an additional nose-down pitching moment at speeds above Mmo, and, indeed, the AFM was revised with the following: “Warning: Do not extend the spoilers, or operate with spoilers deployed, at speeds above Vmo/Mmo due to the significant nose-down pitching moment associated with spoiler deployment. If Vmo is inadvertently exceeded: (1) thrust levers-idle; (2) level wings if required; (3) rotate nose-up not to exceed 1.5 Gs.” Gates Learjet Service Newsletter 49 later issued another warning: “Note: If Mmo is inadvertently exceeded to the point where the airplane seems to be out of control, lower the landing gear.”
The NTSB determined the overspeed was possibly initiated by an unexpected encounter with turbulence, which caused the aircraft to depart the narrow flight envelope boundaries in which it was operating. The Safety Board attributed the accident to the flight crew's improper recovery procedure, and lack of adequate training and experience in the Learjet.
In the early 1990s, the major airlines were developing advanced maneuvers programs. They included nose-high, nose-low/inverted, flight control malfunction and wake turbulence encounters. Demographic data was collected to see if there was any correlation in successful upset recovery performance with pilots’ previous experience in aerobatics or tactical aircraft.
Did those with previous aerobatics or tactical aircraft experience perform better during this experiment? Engineering analysis of the aircraft motions during these upset events actually showed the opposite in a number of the upset scenarios. One of the unusual attitude scenarios that illustrated this unexpected result was the nose-high condition. When pilots had applied the recovery procedure from their previous backgrounds in tactical aircraft, their control inputs resulted in a deepening of the aircraft’s aggravated flight condition.
The explanation is fortunately fairly simple. Transport jets with two large, powerful turbofan engines mounted below the wing produce a very strong nose-up pitching moment when the power is applied. Thus, during a nose-high recovery, application of thrust only makes the situation worse. In fact, the nose-up pitching moment of the turbofan engines can over-power the pitch input of the elevator as its aerodynamic control diminishes with slower airspeeds. The forces and moments created by the powerplants become more and more over-powering as the airspeed decreases, thus having a very negative effect on the aircraft’s trajectory.
So, what is the suggested recovery technique for a nose-high situation? According to the FAA’s Airplane Upset Recovery Training Aid, an extensive document for an aircraft with under-wing engines, the recommended procedure is: (1) recognize and confirm the situation; (2) disengage autopilot and autothrottle; (3) apply as much as full nose-down elevator; (4) roll to obtain a nose-down pitch rate and reduce thrust (no, that wasn’t a mistake; it is italicized for emphasis); (4) Approaching the horizon, roll to wings level, check airspeed, adjust thrust and establish pitch attitude.
The Airplane Upset Recovery Training Aid is an excellent document prepared by an industry working group of airlines, pilot unions and manufacturers. This document appropriately points out that “Aerodynamic principles do not change, but airplane design creates different flight characteristics. Therefore, training and experience gained in one model or type of airplane may or may not be transferable to another.”
In the third part of this article, we’ll discuss control deflections at high altitude and high speed.