[jhb_airlines] Re: Autopilot Help Please

  • From: "Alex - Reheat.org" <Alex@xxxxxxxxxx>
  • To: jhb_airlines@xxxxxxxxxxxxx
  • Date: Sat, 04 Nov 2006 10:02:59 +0000

Good Morning Frank!

Finally! at least one question on here I can answer, rather than it be me asking all the questions! I have replied below in red - hope it shows up on everyones mail client!

(a) When I switch on the autopilot, it appears that the yaw damper and wing leveller also become active, as in this mode the AP continues to fly the aircraft at its current heading and altitude. If I turn on the heading function, the wing leveller turns off, and the aircraft turns to the set heading. From these examples it follows that a single switch change on one autopilot function can trigger off a number of consequent function setting changes. Can anyone give me a list of these inter relationships, or is it not that simple?
Now this is a question of logic, and indeed is one I have asked before at my local maintenance shed. The answer I was given seemed to make sense so I shall repeat it here:

When you turn on the autopilot it is assumed that you no longer wish to manually fly the aircraft, therefore the first thing it does is stick your wings level, reduce any yaw and await further instructions. This is essentially just positioning the aircraft and the automatic flight system to be in the best position possible to react to your next command via the heading or nav systems. The yaw damper bit I will come to presently.
(b) Does the Flight Director just display the cross-hairs as a guide, or has it more uses?
Nope, thats it! it is a "Flight Director" and it just directs the direction of the flight! In earlier aircraft such as the Vickers VC-10 and Early BAc 1-11 these were installed as part of the INS system to show the pilot where the navigation instruments were taking him, the ILS readout and radio nav readouts were all separate gauges so the INS had nowhere to show its intentions (if you like) and so they put it as part of the artificial horizon. Eventually someone realised it would be a good idea to link it to the GPS/INS/FMC/FMS/MDCU as well as the Radio Nav equipment and thus it was installed as a separate unit rather than part of the INS.
(c) What does the yaw damper do precisely?
The yaw damper, when engaged, reduces oscillations and adverse yaw while in straight and level. Earlier airliners (and the Boeing 767 <VBG> ) had lots of cases of passengers being sick due to the gentle oscillation of the aircraft in the cruise, these movements were too small and constant to be corrected by the pilot so the yaw damper was invented to keep the aircraft on as true a level as possible. Just as a note it only controls the rudder and not any other flying surfaces..... which also gives it another use, the yaw damper in a turn will give a perfectly coordinated turn without the need for the pilot to use the rudder - I understand that it is most of the major airlines policies to fly with the Yaw Damper engaged from flaps up to final approach.
(d) For a single engined prop panel, under the engine/power heading I plan to include the settings for mixture, throttle, and propellor percentages, plus the rpm and manifold pressure.
I assume I can just double this up for a piston engined twin, but what do I need to display for turboprops and jets?
It depends on the complexity of the aircraft, for example if you look at the Lear 45 you will see only N1, N2 and Exhaust Gas Temp - those are the basics. For a jet would you need an APU section for when that is on, do you need to show inlet pressures and temperatures? again it depends on the aircraft and engine configuration. Do you have a particular aircraft in mind?
(e) What's the difference between autobrakes, spoilers and air brakes?
Ah now a good question this, an oft misunderstood category. There is a great difference in them and confusing the 3 could lead to nasty circumstances. First of all  Autobrakes is an automatic system to apply the wheel brakes (only!) when suitable on landing. It normally has a variety of settings Such as Min, Med,Max and Wet.
When engaged it will slow the aircraft to a predefined speed before releasing the brakes unless the unit is manually switched off during the roll out.

Spoilers are really just as the name suggest, and are airflow restrictors designed to spoil the airflow over the wing or other flying surface, while providing a physical barrier which slows the aircraft down. The function of spoilers is to reduce airspeed AND lift.

Airbrakes/Speed brakes are normally positioned elsewhere on the airframe (such as the tail or somewhere on the rear fuselage) and are there purely to slow the aircraft down and not reduce lift. A good example of this is the big air brake on the back of a BAe 146 as depicted in this little RC Model:

You can see that it opens just to provide a barrier to the air and slow the aircraft down but is positioned so as to avoid interrupting the airflow of any flying and control surfaces.

To take this a step further there is also Lift Dumpers, which are designed to (you guessed it) dump lift but not slow the aircraft down, although as a secondary effect of reduced lift the speed is of course affected.

To understand these systems further the best representation of these can be found in John Murchisons BAe 146 and the Project Fokker 100, which has them all modeled accurately and separately.

It is also worth noting that most aircraft also have autospoiler/dumper systems for upon landing which work in the same way as the auto brake but for the different surfaces

Sorry if all that just confused you, please feel free to ask any questions on any point and I'll be happy to go into more detail and clarify when needed.


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