B. An INS has the capacity to store data for only nine waypoints at a time. Up to 36 waypoints can be loaded into the PMS. As a result, the PMS provides the INS an extended flight plan capability. To provide the necessary waypoint information to the INS, the PMS transmits data for the FROM/TO waypoints of the current flight leg plus data for the following seven waypoints of the flight plan to the INS. This data is changed as each waypoint is reached and the next leg of the flight plan begins.
C. DME Updating (Fig. 4)
(1) DME updating of present position data from the inertial navigation system is also accomplished through the PMS. There is a choice of either single or dual DME updating. In either case, the PMS uses DME pulse pair information to update the inertial position from the INS. The updated present position is then transmitted to the INS to be used for subsequent horizontal navigation and steering calculations.
D. Triple INS Mixing.
(1) When three inertial navigation systems are installed, triple mixing of INS latitude and longitude results in the use of a mean latitude and a mean longitude to define present position. The triple mixing is accomplished through the PMS CDU, but only when the PMS has waypoint control over the INS.
5. Vertical Steering and Speed Control (Fig. 5)
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A. The PMS provides both vertical steering and throttle control to attain and maintain the desired climb rate, cruise altitude, cruise speed and descent rate. The PMS can be programmed for a descent which will control the airplane to a geographic point at a specific altitude and a specific speed.
B. There are three basic PMS operating modes: climb, cruise and descent.
(1) Climb
(a)
There are three PMS climb modes. These modes are: optimum (minimum fuel speed), Maximum Angle, and Manual (selectable speed). To start any climb mode, the PMS must be engaged and the selected clearance altitude must be set above the airplane altitude.
(b)
Climb, once begun, continues until the clearance altitude is reached. At this point, the PMS levels off the airplane and enters an altitude/speed hold mode even though the airplane may be well below the desired cruise altitude.
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(c)
On IB Airplanes, any one of the three climb modes can be chosen. Regardless of the choice made, the PMS controls throttle settings during a climb to maintain engine pressure ratios (EPR's) at their limits. The choice of climb modes, on the other hand, does affect pitch control. During an optimum climb mode, the PMS regulates pitch attitude to hold an optimum speed which is determined by the PMS based on airplane gross weight.
(d)
On AF Airplanes, any one of the three climb modes can be chosen. Regardless of the choice made, the PMS controls throttle settings during a climb to maintain the N1 percent RPM of the engines at their limits. The choice of climb modes, on the other hand, does affect pitch control. During an optimum climb mode, the PMS regulates pitch attitude to hold an optimum speed which is determined by the PMS based on airplane gross weight. Pitch attitude in a manual climb mode is accomplished to maintain a speed selected by the operator. With the third choice, maximum angle climb, pitch is varied to permit a minimum distance climb to cruising altitude. For such a climb, airspeed will be kept above minimum levels determined by airplane gross weight and altitude through control of the pitch attitude while the throttles continue to command full climb power. During climb mode or other PMS modes, the PMS will automatically restrict speed to 250 knots at altitudes below 10,000 feet.
(2)
Cruise
(a)
The PMS has five available cruise modes. These modes are: Long Range, Maximum Range, Manual (selectable speed), Engine Out, and Optimum Hold.
(b)
In cruise, PMS pitch control holds airplane altitude, plus or minus 120 feet. The 120-foot tolerance permits slight climb and descent motions that are used to smooth out temporary speed changes. Reducing throttle activity in this manner helps conserve fuel. Throttle adjustments, on the other hand, are used during cruise to compensate for longer term effects. An example of such an effect would be the increasing airspeed caused by the continual decrease in gross weight due to fuel consumption.
(c)
Selection of a cruise mode determines the airplane speed during cruise and, hence, fuel efficiency and flight time. Speed during a manual cruise mode is maintained at the value selected by the operator. Speeds during the other possible cruise modes vary. They are calculated, based in part on airplane gross weight and altitude, by the PMS computer. Of these modes, optimum hold is the slowest and the most efficient. However, this mode is useful only for holding patterns near the end of a flight.
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本文链接地址：747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航4(120)
