Page 40 - World Airnews Magazine May 2020 Edition

TECHNICAL

reversals can exceed structural design limits. structures. A prime example of this occurred when a Boeing 747

Also, certification flight tests involve control input in a single axis nearing Anchorage, Alaska, on March 31, 1993, was in the vicinity

and single direction. Control reversals will amplify the loads on the of strong mountain wave turbulence.

aircraft’s structures. The jumbo experienced a rapid 50-deg. roll to the left followed

The Airbus document “Managing Severe Turbulence” provides by a significant yaw. Then came several pitch and roll oscillations.

additional guidance to flight crews of fly-by-wire (FBW) aircraft The severe turbulence created dynamic multi-axis loadings that so

based upon in-depth analysis of severe turbulence events. Airbus exceeded the structural capability of the No. 2 engine pylon that it
engineers discovered that pilots who followed the prescribed separated from the airplane, taking the engine with it.

recommendations for FBW aircraft to keep the autopilot and auto WHAT IS “MULTI-AXIAL” LOADING AND WHY IS IT

throttles engaged were able to minimize the loads on aircraft, as IMPORTANT TO THIS DISCUSSION?

opposed to flight crews who took over manually.

The manufacturer’s Flight Crew Operating Manual recommenda- Its impact on an engine pylon illustrates this phenomenon perfect-

tions are to follow the target speed (which depends on altitude) when ly. Under normal circumstances in flight, the engine is producing

turbulence is encountered and keep auto throttles engaged except if a significant amount of forward thrust, thus its pylon endures a
thrust changes become excessive and keep the autopilot on. direct tensile (pulling) force along its primary axis and is designed
accordingly. Now, consider an abrupt rolling input to the aircraft.

Detailed studies found the autopilot, when combined with the Suddenly, in addition to the tensile force from the thrust-produc-

turbulence-induced motions, limits the aircraft to smaller reac- tion, the pylon experiences a sideways shearing force. This places

tions. In contrast, they also found that pilot pitch-down reaction the pylon into a condition of “multi-axial” force . . . tensile stress

to an initial updraft will accentuate the pitch-down effect as the in one direction and shear stress in another. The combination of

aircraft flies into the downdraft section. stresses acts in a direction that the structural element was not

This increases the negative load factor and increases the risk and designed to withstand originally, and can lead to failure.

number of injuries.
A severe turbulence encounter may lead to excessive high-speed SUMMARY

or low-speed excursions. This will induce autopilot disconnections Due to the risk of reduced structural integrity of an airplane, the

and activation of the appropriate flight control law in an FBW air- NTSB is concerned about deficiencies in inspection procedures.

craft. In the case of a Vmo/Mmo exceedance, a pitch-up would be Implicit in the high-load and inspection formulation was the

commanded to reduce the excursion. presumption that the specified inspections would be adequate to

If excessive low speed occurs, the alpha protection law would identify and address damage caused by any such event.

activate and produce a nose-down movement. In order to keep Canadair and Airbus determined their published inspection cri-

the autopilot engaged as long as possible, flight control software teria were inadequate to ensure safety after the high-load events

modifications have been developed on FBW aircraft. encountered by their airplanes, and that additional broader and

Severe turbulence can induce significant altitude excursions be- more-detailed inspections were required to ensure safety.

cause of the turbulence or as a consequence of triggering the Vmo/ The NTSB wants to prevent inspected but still unairworthy

Mmo protection or the AOA protection. Without the pilot in the aircraft from returning to service after events that exceeded the

loop these protections will place a priority on protecting the target manufacturer’s threshold. Furthermore, the board is troubled that
speed rather than maintaining the trajectory. When either protec- aircraft may be exceeding design and certification standards more

tion law is activated, the autopilot is disconnected automatically. frequently than was known or expected, and recommends all such

The pilot should apply smooth corrections to manage the aircraft events be tracked and evaluated.

trajectory and avoid sudden corrections fighting the turbulence. After encountering significant turbulence or any other event that

According to the industry’s Upset Recovery Manual, if during re- may have exceeded the aircraft’s structural limitations, a captain

covery the application of full lateral control (ailerons and spoilers) is responsible for annotating the fact in the aircraft’s logbook after

is insufficient, it may be necessary to apply rudder in the direction the flight, triggering a mandatory inspection for structural damage.

of the desired roll. The manual warns, “only a small amount of According to Airbus, turbulence can be considered as excessive
rudder input is needed. Too much rudder applied too quickly or when passengers and crew are moved violently against their seat belts
held too long may result in loss of lateral and directional control and objects move around the aircraft. Inspections that are mandated

and cause structural damage.” after flight in excessive turbulence are described in the AMM.

After recovering the aircraft, flight crews must assess any dam- In case of severe turbulence, it is recommended the manufactur-

age that may have occurred. er be informed as well. Note that in some cases the manufacturers

Severe atmospheric turbulence and inappropriate flight-crew have determined that limit loads have been exceeded in portions

control inputs can induce significant “multi-axial” loads on aircraft of the aircraft, thus requiring additional inspections. Q

w
s | Ma
W
World Airnews | May 2020y 2020
orld Airne
— 38 —
— 38 —