Page 39 - World Airnews Magazine May 2020 Edition

TECHNICAL

is required to be designed to sustain the highest loads expected in In both of these events, the operators performed the required

service [‘limit load’] without detrimental permanent deformation inspections as specified in the applicable AMM. However, at the

and any deformation may not interfere with safe operation.” time neither the CRJ nor the A300 AMM included inspections for

Subsequent evaluation of the flight data recorder by Canadair damage caused by high loads due to extreme lateral accelera-

revealed large vertical accelerations ranging from +4.3 to -1.9 neg- tions, such as those encountered by American Flight 903, and the

ative during the turbulence encounter. Canadair AMM did not include inspections for damage caused by

These load factors were outside the certificated design enve- extreme negative vertical accelerations, such as those encoun-

lopes for the wing, pylon and horizontal stabilizer attachment to tered by Comair Flight 5109.

the vertical stabilizer. The inspection procedures in the CRJ AMM

define a minimum positive G threshold depending on the airplane’s PILOT CONTROL INPUTS

weight (+2.5g in this event), above which a visual inspection for Inappropriate control actions by pilots can exacerbate an upset

damage is required. recovery or turbulence encounter, putting additional loads on

No inspection criteria were provided for negative-G or lateral-G the aircraft. Remember that flight at high speed and high altitude

excursions. Canadair subsequently performed an engineering as- produces considerable changes on an aircraft’s stability and

sessment and determined inspections in addition to those already handling qualities. Since air density decreases at higher altitudes,

specified in the CRJ AMM were required to ensure the airplane’s an aircraft’s aerodynamic damping decreases and it becomes more
structural integrity. responsive to control inputs.
On May 12, 1997, an Airbus A300-600 operated by American Flight in the high-speed regime creates high control power, which

Airlines as Flight 903 experienced a stall warning system activation if used improperly, can over-stress aircraft components.

followed by extreme bank angles left and right, and a rapid loss of Over-controlling is a distinct threat at high altitude. For the same

more than 3,000 ft. One passenger sustained serious injuries. control surface movement at constant airspeed, an airplane at

Upon landing, the aircraft was inspected visually for damage 40,000 ft. experiences a higher pitch rate than one at 5,000 ft.

in accordance with the Airbus A300-600 AMM, which specifies because there is less aerodynamic damping.

threshold criteria for positive and negative accelerations in the Therefore, the change in angle of attack (AOA) is greater, creating

vertical axis, but does not specify lateral-G excursions. Damage to more lift and a higher load factor. It takes less force to generate

the engine pylons and engines was noted and repaired before the the same load factor as altitude increases. Erratic and large pitch
airplane was returned to service. inputs, possibly from a startle/surprise effect, can quickly bring

Subsequent analysis of Flight 903’s FDR revealed lateral acceler- the aircraft into an upset. It is imperative that pilots refrain from

ations in excess of 0.4g, resulting in internal loads well in excess of overreacting with large and drastic inputs. Rather, they should

the certificated ultimate loads for the vertical stabilizer attachment smoothly adjust pitch and power to keep the aircraft within the
structure. The stabilizer was removed, and the composite attach- centre of its manoeuvring envelope.

ment lugs were subjected to ultrasonic non-destructive inspection. Aircraft are not built to endure an infinite number of combina-

The results of the right-hand aft attachment lug inspection tions of control inputs. The structural integrity of an aircraft is in-

indicated a delamination. Delamination of a composite structure tended to withstand a pre-defined stress load induced by “normal”

can severely compromise its strength. The NTSB wrote that this control inputs. At speeds higher than manoeuvre speed

type of damage can be caused by extreme loading conditions such (Va), a single large deflection in pitch or roll has the

as those associated with the upset. The vertical stabilizer was potential to generate structural damage or even failure.

removed permanently. At any speed, large aggressive control deflection To page 38
World Airnews | May 2020
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