MD-80 Sec. 4 Page 249
Flight Manual Continental Rev. 05/15/95 #23
FLYING IN TURBULENCE
In severe turbulence, two major concerns naturally arise in the pilot’s mind. One is the concern of imposing excessive structural loads. The other is the concern that airplane attitude may reach undesirable extremes. Neither of these concerns is totally unjustified. On the other hand, the classical treatment of rough air penetration speed has perhaps placed too much emphasis on the structural aspects. Most pilots are well aware that flight through a given set of gusts at higher speeds will produce higher load factors or G’s and a rougher ride for the passengers than a penetration of the same turbulence at a more moderate speed. The admonition to slow down to the rough air penetration speed when entering turbulence has only served to reinforce this concern. Engineering methods for computing the effects of turbulence on structural loads are well known for an airplane in level flight at the time of entry into the turbulence and as a result, the classic discussion of the rough air penetration problem has tended to focus on such calculations and emphasis on the structural significance of high speed entry.
The other major concern, namely that of control, is a much less scientific and less tangible problem and therefore is perhaps lost to some degree in the usual technical discussion. It is important to note here that there is strong suspicion, if not specific evidence, that almost every structural breakup that has occurred in severe turbulence has been accompanied by a prior severe change in attitude and a subsequent combination of stresses resulting from both the recovery maneuver and the severe turbulence. The ride-smoothing qualities of the flexible swept back wing and the high wing loading of today’s modern jet transport make it particularly likely that any structural damage which might occur in severe turbulence will be the result of a severe upset and/or recovery maneuver in combination with the turbulence. Thus, the usual simple calculations in straight level flight are perhaps not sufficiently pertinent to the real problem. Rather, some relationship to an upset condition should perhaps be developed as the basis for defining operating techniques in severe turbulence.
Airspeed
In recent incidents where flight difficulties have been experienced while flying jet transports in severe turbulence, a common factor has been reduction of airplane speed to a value that was close to or below the minimum speed formerly recommended for turbulence penetration. While flight at low speeds is satisfactory in moderate turbulence and may seem more comfortable, there are several disadvantages to flying at low speeds in severe turbulence. First, the airplane is closer to stall buffet and since the angle of attack changes caused by severe turbulence can be high, there is greater chance of encountering strong and alarming buffeting and the accompanying high drag that will cause loss of altitude and tempt the pilot to make undesirable thrust changes.
Second, because the trim changes due to thrust changes are higher in the low speed region, because the airplane is flying on the back side of the thrust required curve at low speeds, and because the trim changes required to keep in trim as the airplane changes speed are greater when flying in the low speed region than when flying at higher speeds, the difficulty of maintaining adequate control is compounded. Also, it is easier for the airplane to be laterally and directionally upset at the lower speeds when turbulence is severe.
Because of the disadvantages of low speed flight enumerated above, it is now considered desirable to emphasize flight at somewhat higher speeds than formerly recommended. The speeds now recommended for all turbulence penetrations tend toward the high side of the range previously recommended. For simplicity, it has been considered desirable to emphasize only one indicated speed, 285 knots or .79 Mach, whichever is less, as target speed with the realization that sizable and rapid variations will likely occur depending on the severity of the turbulence.
The question then arises as to the general practices to be employed in attempting to hold speed within a reasonable variation from the target speed. Moderate variations, either above or below, are of minor consequence, therefore, excessive abrupt or severe control motions should not be required, particularly when it is recognized that some of the fluctuation of the instruments is a result of the turbulence itself and does not necessarily represent a real change in the airplane’s speed or altitude.
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