Descent Performance (Bizjet)
It is also explained in Section 13.1 that only results of the integrated descent performance in graphical form are provided, as shown in Figures 13.16 through 13.18. It is convenient to establish first the descent velocity schedule (Figure 13.16) and the point performances of the rate of descent (Figure 13.17) down to sea level (this is valid for all weights; the difference between the weights is ignored). When readers redo the calculations, there may be minor differences in the results.
The related governing equations are explained in Section 13.4.3, which also mentions that the descent rate is restricted by the rate of the cabin-pressurization schedule to ensure passenger comfort. Two difficulties in computing the descent performance are the partial-throttle engine performance and the ECS pressurization capabilities, which dictate the rate of descent that, in turn, stipulates the descent velocity schedule (these are not provided in this book). Instructors may assist in establishing these two graphs, which – in the absence of any information – may be used. The following simplifications are useful.
The first simplification is in obtaining the partial-throttle engine performance, as follows:
1. The zero thrust at idle rpm is at about 40% of the maximum rated power/ thrust.
2. The maximum cruise rating is taken at 85% of the maximum rated power/ thrust.
3. The descent is carried out from 40 to 60% of the maximum cruise rating.
The second simplification is provision of the descent velocity schedule for the ECS capability.
In the industry, the exact installed-engine performance at each partial-throttle condition is computed from the engine deck supplied by the manufacturer. Also, the ECS manufacturer supplies the cabin-pressurization capability, from which aircraft designers work out the velocity schedule.
The inside cabin pressurization is restricted to the equivalent rate of 300 ft/min at sea level to ensure passenger comfort. An aircraft’s rate of descent is then limited to the pneumatic capability of the ECS. A Bizjet is restricted to a maximum of 1,800 ft/min at any time (for a higher performance at lower altitudes, it can be increased to 2,500 ft/min). The descent speed schedule continues at Mach 0.6 from the cruise altitude until it reaches the approach height, when it then changes to a constant VEAS = 250 knots until the end (for a higher performance, it can be increased to Mach 0.7 and VEAS = 300 knots). The longest ranges can be achieved at the minimum rate of descent; this requires a throttle-dependent descent to stay within the various limits.
It is convenient to establish first the point performances of the velocity schedule (Figure 13.15a) and the rate of descent (Figure 13.15b is for all weights; the variation is minor). The descent is performed within the limits of the passenger comfort level. However, in an emergency, a rapid descent is necessary to compensate for the loss of pressure and for oxygen recovery.
An integrated descent performance is computed in the same way as the climb performance; that is, it is computed in steps of approximate 5,000-ft altitudes (or as convenient) in which the variables are kept invariant. (Computation work is not shown herein.)
Figure 13.16 plots fuel consumed, time taken, and distance covered during the descent from the ceiling altitude to sea level. When readers redo the integrated descent performances, there may be minor differences in the results.