Hello Gums, please return from lurk mode.
To answer your original question, I apply the old adage, to try to explain it in such a way, that your granma would understand it. Following is the way I explained it to my son when he was in flightschool.
AUTOFLIGHT
A summary for the subject assignment Autoflight.
In this assignment, a lot of terms are used, that may cause confusion if they are not properly explained: autopilot, flight management, autothrottle, autothrust, flight guidance, flight control computor, flight control panel, flight control unit, etcetera, etcetera.
An autopilot is a simple apparatus, that can somewhat fly an airplane.
An aircraft is controlled through the use of elevator, ailerons, the rudder and thrust. Furthermore there are (leading and trailing edge) flaps and speedbrakes.
Autopilots normally provide only control with the elevator and ailerons, they have a pitch channel and a roll channel. In exceptional cases they may also provide yaw control with the use of the rudder.
The control of the gas (thrust / jets or power / props) is managed by the autothrottle (or autothrust), nomenclature is more a matter of taste of the manufacturer. There is a certain relation between autothrottle and autopilot functions, but that can probably not be called real cooperation.
The flaps (and slats), speedbrakes and landing gear are never controlled by an autopilot, all configuration changes are made manually by the pilot.
AUTOPILOT
Basically an autopilot performs only some very simple functions:
Attitude hold - the attitude is maintained (pitch and/or roll)
Steering towards a certain value -
Pitch/ maintaining an altitude
a speed
a rate of climb or descent
Roll/ maintaining a heading (Heading Hold)
steering towards a heading (Heading Select)
Furthermore there are complicated modes whereby steering is based on the deviation of a radio signal, for instance pitch for following the ILS Glideslope and roll for following the ILS Localizer. Also the final phase of the autoland function is complicated – signals from the radio altimeter are used to flare the aircraft and accomplish an acceptable touchdown. On the runway nosewheel steering is integrated to stay on the centerline of the runway.
A factor that can complicate the picture, is the description of what happens at the moment that the autopilot is engaged: that is not necessarily during a static situation, it can also be during a dynamic situation.
Let’s say you select Heading Hold during a turn – during the rollout of course the heading will change a little bit. Should the autopilot now turn back to the heading that existed at the moment of engagement, or should it continue on the heading that exists at the moment of wings level (which could be defined as reaching a bank angle of less than 5 degrees)? That sort of “submodes” of course is described in the documentation.
For a mode such as Heading Hold, it is not necessary to set a value anywhere. Of course, a source of information is needed (for instance the Inertial reference System).
For a mode such as Heading Select of course it is necessary as pilot, to set the desired heading. That is accomplished with a knob on a panel that is usually located near the windscreen (within reach). The name of that panel is again a matter of taste of the manufacturer – Mode Control Panel (MCP) or Flight Control Unit (FCU). It is also possible to let the desired heading be determined by the navigation system. The autopilot is then “controlled” by the Flight Management Computor, or the Flight Guidance System, or whatever name they have given to the equipment. The mode will then have a name like NAV (Navigation) or LNAV (Lateral Navigation).
The pitch mode will normally work
in conjunction with the autothrottle. Note specifically that I do not say that they cooperate – they work each in splendid isolation, but their functions are selected in such a way, that there is a logical relation.
The gascontrol can operate in a thrust mode or in speed mode.
In thrust mode, a certain level of thrust is maintained (e.g. take-off thrust, but idle thrust is also possible).
In speed mode, thrust is varied as needed to maintain a certain speed.
The pitch control can vary the pitch attitude to maintain a certain altitude (or a specific rate of climb or descent, or even an ILS glidesope, in other words, the vertical value), or it can maintain a certain speed (that is an horizontal value).
The inter relation now is as follows: when the pitch control steers towards the vertical value, then the autothrottle takes care of the speed.
When the autothrottle maintains a certain thrust, then the pitch control will take care of the speed.
In that way, altitude changes are taken care of
: when you are flying at a certain altitude (pitch in altitude control and autothrottle in speed mode) and you want to climb, then autothrust goes to thrust mode and increases to climb thrust. Simultaneously, pitch mode switches to speed mode. The increased amount of thrust will want to accelerate the aircraft. Pitch control will react by pulling up the nose, executing its task to maintain speed constant. As a result, aircraft will climb.
When approaching the new desired altitude, the functions are swapped again
: pitch will maintain the altitude and autothrottle must maintain speed constant. Because pitch will have to decrease attitude in order to maintain altitude, speed will increase when thrust remains at climb thrust. So, autothrust will reduce thrust in reaction to the level off by pitch control.
Of course you will read about “altitude capture modes”, those are temporary modes for the transition between the level change mode and level flight. The autopilot will calculate a nice transition between those two phases and tries to fly that as smoothly as possible, otherwise you get rough movements and sick passengers.
The mode names again depend on fashion: Altitude capture is annunciated by one as ALT CAP, by the other as ALT*
The setting of speed can be done by the pilot, with a knob on the glareshield panel (MCP or FCU), but can also be delegated to the Flight Management System. That sytem can maintain pre-programmed speed restrictions in Standard Instrument Departures (SID’s), or the standard limit of 250 kts below 10.000 ft. During the climb, economic climb speed, depending on the entered cost index
; at altitude, the desired cruise speed, based on cost index and actual wind conditions, etcetera.
The name of the pitch mode again depends on the fantasy of the manufacturer – VNAV (Vertical Navigation), CRZ (Cruise), or PROFILE, you name it.
For descent the operation is as during the climb, only then of course, the autothrottle will choose idle thrust.
There are also somewhat more complicated level change modes – for relatively small altitude changes it is possible that the autothrottle will go to a sort of half gas value, this in order to prevent extreme thrust changes (jet engines suffer most from large temperature changes of the hot components).
Boeing for instance has the Flight Level Change mode (FLCH), where the thrust computer calculates approximately what thrust is needed to reach the new altitude in 2 minutes. If it is for a climb, then of course for a large altitude differential you will run into the climb thrust limit; if it is for a descent, then for a large altitude differential you will reach idle thrust. In case of large altitude differences, of course you will need more than 2 minutes, but in case of small altitude changes (1.000 or 2.000 ft), you will achieve that the engine life is spared and the flight movements are smoother.
For the descent there also is a sort of PROFILE mode: the descent is then flown in principle with idle thrust, but programmed altitude and/or speed restrictions in the arrival route will then be respected.
In that way, it is possible that on certain stretches, the autothrottle will increase thrust above idle, when the “desired” segment is too flat for an idle descent at the desired speed.
Complicated? For instance, when approaching from Germany to Amsterdam, waypoint NORKU (on the border) is restricted to BELOW FL280, speed 280 – 300 kts. Later on, at the TMA boundary, BELOW FL100, ABOVE FL070, speed 250 kts. At ILS interception point NOT BELOW 2.000 ft.
It is also possible that a segment is so steep that, with idle descent, maintaining the path leads to an increase in speed. It is possible that the system than advises with a message “DRAG REQUIRED”, or it may even anticipate the problem with a message “TOO STEEP PATH AHEAD”. What you then do as a pilot is up to you - perhaps you have the bigger picture that everything will nicely settle without any fuss, perhaps those messages open up your eyes that action is necessary, anyway, as stated earlier, speedbrakes are not under the authority of the autopilot.
Note that all the modes that have been mentioned, are indicated on the Primary Flight Display, just above the Attitude Indication in the so called Flight Mode Annunciation (FMA), green letters, every change highlighted by a box drawn around the change for some 10 seconds.
Some snags are possible for the unwary. Mostly those snags are in the combination of manual flight and autothrottle, but not all.
The autopilot calculates what it would do, when in control of the aircraft. With the A/P in command, it does as it calculates and as long as all systems are serviceable, A/P and A/T do a nice job of flying the aircraft, if need be, from shortly after take-off all the way to rollout after landing.
When the A/T is tits up (case, almost stall of Thomson at Bournemouth), or when it is OK by itself, but fooled by another system input (case, crash of Turkish at Amsterdam), then you will see that the A/P happily maintains ILS Glideslope, with nobody taking care of the speed.
When all systems are OK, but the pilot does not check that he has selected the correct mode, then you may see the aircraft happily tracking down the ILS at ever increasing speeds, rather than starting the Go Around, when the pilot has increased the thrust manually, but failed to trigger the Go Around mode and failed to notice the Flight Mode Annunciation that still is in LOC G/S !!! (multiple occurrences of Air France at Paris De Gaulle with anything from A-320 to B-777.
When the A/P is not in command, its calculations can be shown as output of the Flight Director – if you follow the pitch and roll steering commands of the FD, then you will follow the path that the A/P would have flown.
Remember, the division of tasks is the same as described for the A/P modes – A/P and A/T swap roles, but they work in splendid isolation. If you do not follow FD commands at a time that A/T is in a thrust mode, actually nobody is taking care of the speed !!!
A/T may have a bottom function to kick in regardless when angle of attack reaches a threshold value. This function may surprise pilots and also, the pitchup effect may exceed A/P authority if the A/P had trimmed to a very aft setting in the attempt to maintain G/S at a very low speed (case, Thomson at Bournemouth, even though thrust selection there was associated with Go Around).
The just mentioned A/T protection feature may also be defeated by the A/T being in THR HOLD mode – a situation that may be associated with FLCH mode: after the A/T has set the calculated thrust value, it disconnects or goes dormant to allow the pilot to set a different amount of thrust without having to constantly fight the A/T.
Note that THR HOLD is annunciated in the FMA !! This gotcha got the Asiana crew.
Bottom line
– take note of what the aircraft is telling you
- monitor what the aircraft is actually doing (e.g. speed)
- take action if you don’t like what is happening
With regards gums, to your question about use of AoA (indicators) in commercial big jets – here is a reference to an excellent article on that in the Boeing AERO magazine, edition 2000 quarter 4, find it via link
AERO
As the edition is already quite old, the text and pictures are not anymore collated, but it still is well worth the effort to retrieve and read it all.
With regards to your questions about use of A/T in engine out situations – depends on make and model and also on what options companies have been willing to buy into. Sometimes engine out means manual thrust only certified, sometimes A/T and Cat 3a ILS to autoland is still certified.
Use of rudder in engine out case – same deal. Some aircraft provide yaw control with rudder always, some rely on manual rudder until multiple autopilots are engaged for a coupled approach.
Reaction of aircraft also depends on whether FBW or not – if engine out causes yaw, then roll coupling will occur in non FBW system, but counter aileron (and skidding flight) is result in FBW system.
Hope this will clear up matters for you a bit, just post if any questions remain.
Check six.
Edit - only some layout items