Airspeeds

Limitations

7 CHAPTER

As you know, every airplane is designed to operate under a set of limitations: airspeeds, engine parameters, temperatures, fuels, weight and balance, and various structural and system limitations. Many are depicted in the cockpit via placards and instrument markings, and others are found in the AFM (Aircraft Flight Manual). When “checking out” in turbine aircraft, many limitations for each aircraft type must ultimately be memorized by the pilot. In this section, we take a general look at some of the most common types of limitations, with emphasis on those uniquely found in turbine aircraft.

Airspeeds

Limiting airspeeds for turbine aircraft are, in many cases, similar to those of their piston-engined relatives. Best angle and rate of climb speeds are defined similarly (though using different terms), as are stall and minimum single-engine control speeds. However, there are some interesting variations at the higher end of the speed range. There is no redline on the airspeed indicator of a turbine aircraft. These planes are limited by VMO and MMO (both lumped under maximum operating limit speed), rather than the VNE (“never exceed speed”) found in piston aircraft.

VMO is a structural limit designed to prevent airframe damage from excess dynamic pressure. MMO protects the aircraft from shock wave damage as the aircraft approaches the speed of sound. (See “Aerodynamics of High-Speed/High-Altitude Aircraft” in Chapter 15.) The values of both VMO and MMO vary with altitude. VMO governs as maximum operating speed at lower altitudes, while MMO is most restrictive at higher altitudes. Since it would be impossible to depict all this information on the airspeed indicator using a fixed redline, maximum operating speed is shown on turbine aircraft by a red and white striped needle commonly referred to as the barber pole. On the airspeed indicator of a turbine aircraft, the barber pole moves automatically to display correct maximum operating limit speed for conditions at any given moment (Figure 7.1).

At jet speeds, cruise is described by Mach number, rather than in knots or miles per hour. Mach number describes airspeed relative to the speed of sound, which is represented as Mach 1. An aircraft traveling at Mach 2 is traveling at twice the speed of sound, while one traveling at 0.85 Mach has attained 85 percent of the speed of sound. (See “Aerodynamics of High-Speed/High-Altitude Aircraft” in Chapter 15.) Aircraft Mach speed is displayed on a Machmeter, usually associated with the airspeed indicator.

Like its piston counterparts, each turbine aircraft is assigned a maneuvering speed (VA), which limits the airspeed at which controls may be fully and rapidly deflected. However, a turbine aircraft additionally has a specific turbulent air penetration speed (VB), which offers maximum-value gust protection.

In addition, many aircraft have specific airspeed limits for operation of various systems. For example, some Beech King Airs have limiting airspeeds pertaining to operation in icing conditions. There’s a maximum airspeed for use of engine ice vanes and one for effective use of windshield heat. The same aircraft are also assigned a minimum airspeed for climbs in icing conditions, in order to prevent underwing icing.

Large aircraft landing gear systems often have a few additional airspeed limitations you’ll need to keep in mind. The landing gear extended speed (VLE) is the speed limit at which it is safe to fly with the landing gear secured in the fully extended position. The maximum landing gear operating speed (VLO) is