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For the reasons previously enumerated, emphasis should be placed on flying on the high side of the target speed rather than the low side. However, it is considered highly undesirable to chase airspeed either with elevator or throttle manipulations since these efforts are usually ineffective and, as will be pointed out later, attitude should be the principal flight reference in turbulence.
Attitude
Flying under extremely turbulent conditions requires techniques which may be contrary to a pilot’s natural reactions. Rapid and large aileron control inputs are permissible to hold the wings level, but in extreme turbulence, pitch attitude must be controlled using only small to moderate elevator control inputs to avoid overcontrolling or overstressing airplane structure. The natural stability of the airplane will work in a direction to minimize the loads imposed by turbulence. The pilot should rely, to a major extent, on this natural stability and not become too greatly concerned about pitch attitude variations. Since there is always the uncertainty of the direction, timing, and size of the next gust, it is often better to do nothing at all than to attempt to control airplane pitch attitude too rigidly. The moderate control inputs that are considered desirable will not always allow very precise attitude control. Ideally, elevator control should be applied smoothly in a direction to resist motions away from the desired attitude, and the elevator should be returned to neutral when the airplane is progressing toward the desired attitude. The above described technique will help prevent overcontrolling, and will result in less “G” loads than a technique which very closely controls pitch attitude.
Pitch attitude should be controlled solely with the elevator, never with stabilizer trim. Rapid changes in airspeed and attitude due to extreme gusts and drafts make stabilizer trim difficult to apply effectively. Also, any updraft or downdraft which might tempt the pilot to change trim can be expected to reverse itself in the next few seconds. If trim has been applied to counter the first draft, the second draft, which will likely be in the opposite direction, will exaggerate the out-of-trim condition. It is therefore considered desirable to leave the stabilizer trim alone in severe turbulence.
Thrust
Once the proper thrust setting for the speed recommended for penetration is achieved, it is generally undesirable to make thrust changes during severe turbulence encounters. Large variations in airspeed and altitude are almost certain to occur in severe turbulence, and simple rules of thumb for setting thrust are not universally applicable for all altitudes and weights. The most desired thrust setting in one which will provide near level flight at the recommended penetration speeds of approximately 80 to 85% will be satisfactory. The most important objective is to obtain an initial thrust setting reasonably close to the correct one. Disengage autothrottles if in use.
Altitude
Because of the very high velocity updrafts and downdrafts in severe turbulence regions, large variations in altitude are almost certain to occur. Too much concern about these variations will merely lead to excessive control manipulations, causing large “G” load variations and unwanted airspeed excursions. Altitude should be allowed to vary within reasonable bounds. At high altitude or during high-speed cruise at intermediate altitude, turbulence encounters may produce high speed buffeting. The airplane has been flown into the high speed buffet regime many times during flight tests in the process of determining and evaluating its qualities under these conditions. No unusual flight characteristics have been noted. However, to the uninitiated, the buffeting or shaking might be disconcerting, being somewhat similar in nature but more severe than the shaking that occurs under some conditions when speed brakes are extended.
When experience in combination with severe turbulence, these effects might easily be incorrectly diagnosed as increased severity of the atmospheric disturbance, and result in an exaggerated assessment of the seriousness of the situation. Experience to date has shown that severe turbulence encounters at high altitude have caused positive G’s as high as 2.5. However, it is believed that if the recommended attitude control procedures are followed, high load factors need not be imposed.
Even though these procedures are used, an occasional encounter with high-speed buffeting in unexpected severe turbulence may be unavoidable above 30,000 feet. Such an occurrence should not cause great alarm nor be misinterpreted as a low speed stall with an accompanying rapid pushover for recovery, since any such action might aggravate the buffet situation by merely increasing the Mach number.
This tendency to encounter high-speed buffeting in severe turbulence is increased with increasing altitude. It is therefore apparent that climbing in an attempt to avoid an area of expected severe turbulence would lead to this type of buffeting difficulty if the turbulent region could not be completely topped.
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MD-80 Flight Manual 麦道80飞行手册 2(49)
