PMDG 747-400 Type Rating Course Autothrottle Care and Feeding

From PMDG Ops

Contents

Introduction

Based upon what we are seeing in the postings around various sites it appears that there are some misconceptions and confusion surrounding the autothrottle and it's correct operation.

As we review reports from users about "overspeeding in VNAV" and "airplane not maintaining speed in the climb" it is clear to us that some of the realism updates we added to the A/T system for Version 1.10 have created a bit of a tar-baby for some users, so we thought a quick primer on autothrottle use would be of value.

Terms

Abbreviations you'll need for this lesson:

  • AGL: Above ground level
  • AFS: Automatic flight system
  • AT: Autothrottle system
  • EEC: Engine Electronic Control
  • FMA: Flight Mode Announciator
  • MCP: Mode Control Panel
  • TO/GA: Take-Off / Go-Around button (mode)
  • VC: Virtual Cockpit

Autothrottle

NOTE FOR ALL USERS

Many users without actual airliner experience sometimes get confused when it comes to the autothrottle. Here are a few basic rules to live by:

  1. AutoThrottle ARM switch on the MCP: This is a two position switch ARM and OFF. If the switch is OFF you are controlling throttles manually. If the switch is ARMED you are still controlling the throttles manually UNLESS you see a thrust mode displayed on your Primary Flight Display.
  2. To command the Autothrottle to assume control of the thrust, you must set the Autothrottle switch to ARM, and then you must also activate a specific thrust control mode. The ONLY way to tell if the autothrottle is ACTIVE is to watch for a thrust mode annunciation on your Primary Flight Display.
  3. The active AT mode is shown on the Primary Flight Display using the Flight Mode Announciator (FMA). The FMA is located on your PFD across the top of the Attitude Indicator. The FMA is broken into three parts: 
THRUST MODE | ROLL MODE | PITCH MODE

If the THRUST MODE section of the FMA is blank, then the autothrottles are not controlling the engine thrust and/or aircraft speed and you will receive no speed protection from the autopilot. Conversely, if the thrust mode displays a green (Active!) mode- then this is your indication that the Autothrottle is indeed controlling your throttles!

Example:

If your FMA looks like this:

INSERT IMAGE

You are in control of the throttles... Even though your MCP might look like this:

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If you see a thrust mode displayed in your FMA, then the autothrottle is doing the work for you:

INSERT IMAGE

Remember: If there is no thrust mode displayed on the FMA - YOU ARE IN CONTROL OF THE THROTTLES!

Operation Notes

The autothrottle (AT) system is not very complicated. It has only THREE POSSIBLE STATES:

  • Disarmed and Disengaged
  • Armed and Disengaged
  • Armed and Engaged

We'll review each of these states:

  1. Disarmed and Disengaged (AT switch OFF)
    • The AT FMA entry is blank.
    • Pilot has full throttle authority.
    • The AT system cannot be engaged by any automatic or manual selection of any AFS pitch mode (VNAV/FLCH, etc) or by pressing any of SPD or THR or TOGA buttons.
    • YOU ARE OFFERED NO SPEED PROTECTION (Fast or Slow).
  2. ARMED and Disengaged (AT switch ON / no active mode)
    • The AT FMA entry is blank.
    • Pilot has full throttle authority.
    • Automatic pitch mode changes (e.g. VS to ALT HLD) will have no effect on the AT state -- the AT will remain armed but will not command thrust changes.
    • YOU ARE OFFERED NO SPEED PROTECTION (Fast or Slow).
    • The AT system can be engaged (or re-engaged if previously disconnected):
      1. By manual selection of any pitch mode
      2. By pressing the SPD or THR buttons if the active pitch mode is not VNAV or FLCH.
      3. By pressing the TOGA button if the active thrust limit is respectively TO or GA.
  3. Armed and Engaged
    • The AT ARM switch is ARMED.
    • A thrust mode has been selected.
    • The AT FMA entry will show any of the following : SPD / THR REF / THR / IDLE / HOLD
    • With the exception of HOLD, thrust is now controlled by the AT system.
    • If HOLD is announced, throttle servos are disconnected and throttle levers will *not* be manually repositioned by the AT system. (HOLD is displayed during takeoff and during idle power descents. See notes further below.)

The AT system can be disconnected:

  1. By pressing the AT soft disconnect button -- right click on the TOGA screw (the screw on the MCP above/left of the AT ARM switch) while airborne in the 2D panel or the throttle disconnect button on the throttle levers in the VC.
  2. Setting the AT arm switch to OFF.

Making it Real

If you have some free buttons on your flight control hardware, you can use the PMDG menu/General/Key Commands to map a key to activate the autothrottle and another key to disconnect the autothrottle. You can then map these key commands to your hardware using whatever software came with your flight controls. Once this is completed, a simple button press will activate TO/GA control of the autothrottle, and a single button press will disengage the active thrust mode controlling the throttles. This most closely resembles the controls you would have under your fingertips in the actual airplane.

Supplemental Notes

The AT system will be automatically disconnected:

  1. Once reverses are extended
  2. If more than two engines are shut down / failed
  3. If any EEC module fails. NOTE: If any EEC module fails, the AT system can be subsequently re-connected if *all* EEC's are set to ALTN. In the latter case, however, automatic thrust limiting at maximum N1 is *not* operative. (Yes- we modeled this....)

SPD button is operative once above 400ft AGL and the active mode is not VNAV or FLCH

THR is inhibited below 400ft AGL after takeoff. If the active thrust limit is not TO and not GA, pressing THR will result in the active thrust limit to be reset to CLB or CON in case of engine inoperative. Additionally, if the active AFS mode is not VNAV or FLCH then the active thrust mode will become THR REF (max N1).

PMDG Model Implementation Notes

Checking the AT Override option through PMDG Menu/Options/AFDS allows manual throttle control when the active AT mode is HOLD. HOLD is announced during take off when speed is greater than 65 knots or during idle descents after the AT systems retards throttles to idle stop.

INSERT IMAGE

When HOLD is the active A/T mode it is possible for you to manually adjust the throttle position (and engine thrust output) by moving the throttles on your own. This is particularly useful in the event of a rejected takeoff above 65 knots. Smoothly close the throttles and you will override the takeoff thrust applied by the autothrottles.

Variable minimum engine idle control

The EECs will limit the minimum idle speed of the engines in the following conditions:

  • High altitudes
  • Flaps extended
  • Engine anti-ice ON

HOLD Mode with AT Override option checked

Users are advised that in order to simulate advanced EEC features such as engine idle control within the MSFS environment, allowed minimum throttle position is variable (see above). Hence, during idle descents through approximately 28000 feet in VNAV SPD or VNAV PTH or FLCH (down), the user will be required to *manually* momentarily reposition the joystick throttle to idle or press F1.

Aborted take off

Take off can aborted by right clicking on the TOGA screw on the 2D panel. If the AT Override option is checked then by simply manually retarding the throttles to idle will trigger RTO behavior.

Touchdown/Landing

The AT system will not automatically retard throttles during manual (AP disengaged) or non-ILS landings.

Common User Errors

We are seeing a number of users report over/underspeed conditions that result from errors when configur common errors from pilots:

Error Most Commonly Seen: The pilot assumes that because the airplane is in VNAV and the autothrottle is armed that you will have under/overspeed protection. If there is not an active thrust mode displayed on the FMA, however- this is not the case- and the airplane will over/underspeed unless the pilot manually controls the throttles.

Example of Most Common Error

In this image you can see that the pilot has the autothrottle ARMED and the airplane is being flown in VNAV. The pilot expects that the autothrottle will provide overspeed protection during the descent.

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However... The FMA does not show an ACTIVE AUTOTHROTTLE THRUST MODE. As a result, the airplane will simply accelerate/decelerate as it sees fit.

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Since there is no active autothrottle mode, there is also no overspeed or underspeed protection will be provided by the throttles. When we have reviewed overspeed complaints from pilots, we have found that this is most often the case.

To correct this problem, it is necessary to engage the autothrottle using TO/GA, SPD, THR (depends upon pitch mode in use) or VNAV, FLCH if desired.

Here endeth the lesson...

(Special Thanks to Dr. Vaos, who is largely responsible for programing the autothrottle behavior and logic, including the significant amount of math that goes on behind the scenes!)

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