Airbus Throttle Positions

[shon7]

I was reading in a magazine (airliner world or airways) about an Airbus flight and it said that the throttles on the Airbuses have only three positions. How does this work? For instance - what if you are low on approach and want to apply more power?

[tired]

If you have the autothrust engaged, then there are 3 detents - Climb, Max Continuous/Flex and TOGA. If you take the Autothrust out, which you can do at any time in the flight, then the thrust levers work just like the T/Ls on any other aircraft.

To explain the 3 detents a little more, since this is what you're asking about - at take off, the T/Ls are placed either in TOGA if you're doing a max power T/O, or in Flex/Max Continuous detent if you're doing a Flex power T/O. When the levers are in either of these detents the FADEC will give you the max power available for that selection (It depends on the ambient conditions). When you reach Thrust Reduction altitude you move the levers backwards to the Climb detent, and you will then get Climb power. The levers remain in this detent until you reduce thrust to idle in the flare during the landing - this means that the FADEC will give you whatever power is necessary to achieve what you are trying to do - climb power for the climb, or whatever thrust is necessary to achieve your cruising Mach number, or desired rate of descent or whatever.

If an engine fails, then the levers on the remaining engines are moved back into the Flex/Max Continuous detent, and you will get Max Continuous power. If you have to do a go-around at the destination, you put the levers into the TOGA detent and you get all the power available.

This is all pretty standard stuff for an Autothrust system, the main difference between Airbus's system and Boeing's is that on a Boing the levers move constantly as the FADEC adjusts the power, whereas on an Airbus the levers remain static unless the pilot moves them as described above - to see what the FADEC is doing in an Airbus you have to monitor the N1 gauges.

As I said in the beginning, the pilot can deselect the Autothrust and move the levers manually at any time if he feels like it.

Hope that all makes sense - have just returned from a night spent flogging across the pond so my brain is not too active at the moment. :)

Ah, have just read your question about what you do if you're too low on approach and need more power. If you leave the Autothrust engaged it will (eventually!!) add the required power - that's what Autothrust systems are for. If you've disconnected it for the approach (some of us do sometimes, just ot prove to ourselves that we still can ;) ) then you add the needed power yourself - just like any other aeroplane. :)

(Edited to add the last paragraph).

[shimmydamper]

if you get low on energy/slow and you want to add power faster,
with autothrust you can go momentarely forward of the climb detent. this will give a increase thrust faster then waiting for the autothrust, especially in gusty conditions when the headwindcomponent changes constant, this in turn will change the "grounspeedmini"

above mentioned proc is mentioned in the fcoms 3.04.70p2 for the a330

s.

[tired]

sd - yes, you're right, same thing on the 340. Personally I prefer to take the A/T out in gusty conditions and do it myself because of the A/T's slow responses, but that's personal preference.

[NWSRG]

To add another question, when an Airbus is being set up for take-off, does the aircraft need runway info, so that it can pre-select take off power? If you only have a detent, how do account for different runway lengths? I'm only an amateur, so I hope I'm not missing anything obvious.

[fantom]

the required thrust is extracted from the performance page (paper) for the runway in use. it is either max (TOGA thrust) or Flex - see lesson on the use of assumed temperature.

Airbuses do not have throttles - they are fitted to cars. we use thrust levers.

:p

[mutt]

NWSRG,

I think that you are getting confused with the use of one position, this doesn’t mean that the aircraft is using the same amount of power each time, as tired explained at take off, the T/Ls are placed either in TOGA if you're doing a max power T/O, or in Flex/Max Continuous detent if you're doing a Flex power T/O. When the levers are in either of these detents the FADEC will give you the max power available for that selection (It depends on the ambient conditions).

This is pretty similar to Boeing aircraft in that the amount of thrust required is calculated for every takeoff based on the weight/runway length/temperature/altitude/thrust rating. The difference is that on Boeings the power levers are set to a variable position by the crew or A/T, on an Airbus the crew just set the power levers into a detent.

They are completely different philosophies of doing the same thing, you will find people who love or loathe both methods. :)

Mutt.

[tired]

NWSRG - yes, apologies, my initial post didn't really explain things all that well. (I did warn you my brain was in "sleep" mode after a night spent flogging across the ocean! :) )

As mutt and fantom have explained, before each take off we consult the performance charts for the specific runway we will be using. Because these charts are specific to the runway, they take into account it's length, it's height above sea level and any obstacles that will affect the take off path in the event of an engine failure. The other 2 variables that affect the T/O are ambient temperature and aircraft weight. Knowing what the temperature is allows us to work out how much weight we can lift off the runway - seeing as we know how much the aeroplane weighs when it's empty, we can then work out the total amount of fuel and payload that can be loaded. (And despite what you may read in the press, in most companies the Captain ALWAYS wins the argument if it comes down to trading payload and fuel in order to meet the weight limit - it's always the freight that stays behind. ;) ).

Once we know how much the aeroplane is going to weigh at T/O we can then consult the charts to see whether it's possible to use Flex power for T/O, or whether we will need to use TOGA. In other words, let's say that the temperature allows us to T/O at 250 tons, but the aeroplane will only weigh 230 tons, we will then be able to Flex the T/O - use less than full power and still safley get airborne, or stop on the runway, even if we lose an engine at the critical moment. The chart will also give us the V speeds to use during the T/O - V1, VR and V2.

As far as the actual power that is set, this also depends on the ambient conditions - basically it depends on the air density, which is affected by temperature and height above sea level, and also by humidty etc. The Airbus is a very clever machine (which is why I fly them - designed for dumb pilots :) ) and the FADEC will sense all these factors via built in sensors and automatically compute just how much thrust the engine can develop. When we put the T/Ls into the appropriate detent, the FADEC will set the corresponding thrust by itself, unlike the Boing where the pilots have to set the thrust manually - that's so much hard work. :)

I hope this makes things a bit clearer?

t

[AhhhVC813]

The bit about Boeing pilots setting the thrust manually is not neccessarily correct. On any Fadec driven Boeing with autothrust, which is armed for takeoff, the thrust levers are driven to the correct graduated or max power position by pressing the TO/GA buttons on the thrust levers. Once the power is set, the thrust levers are kept in that position until the time comes to set climb power.

[NWSRG]

Gents,

Thanks for all your comments - but still a bit confused. I understand the FADEC setting the power required, even with one physical thrust lever position. But runway length has to come into the equation. So is this something picked up by the pilot from a chart, and inputted into the aircraft systems manually? If so, what piece of info does the pilot enter? Also understand the aircraft assessing ambient temperature (and weight?) automatically. Can the aircraft also determine the runway it is lining up on? (Obviously heading no problem, but can an aircraft yet distinguish between, say, 27L and 27R?)

Again, please forgive a keen amateur!!

[mutt]

NWSRG,

The procedure for calculating takeoff weights and speeds varies from airline to airline
as well as aircraft to aircraft, my intention with this is to give you a
rather simplistic indication of what happens.

Crews/dispatchers are supplied with charts which provide limiting takeoff weights
for specific runways, this one is for a Boeing aircraft and doesn’t show V-speeds.
(Paste it into Excel and you will get better formatting)

DEG C LIMIT 14L 14R 32L 32R
-15 2874 2460# 2390# 2760F 2874#
-10 2874 2458# 2388# 2737F 2874#

ADD KG/KT HEADWIND 190 210 0 0
SUB KG/KT TAILWIND 870 910 1990 440
MIN FLAP RET HT-FT 1150 1130 800 800
RUNWAY LENGTH-FT 11483 7776 7776 11483
RUNWAY SLOPE-PCT 0.51 0.72 -0.72 -0.51
CLEARWAY-FT 0 0 984 0

If you look at the columns for 14L and 32R, these are the opposite ends of the same 11483 feet runway,
but have different limiting weights due to the obstacles. The actual weight must be equal to or less than
this limiting weight and is entered in the FMS to obtain the V-speeds and power settings.

Airbus (and some Boeings) use a slightly more complex method of calculating Vspeeds, so these are usually
shown on the takeoff chart with the limiting weight.

The aircraft knows exactly where it is for the purpose of navigation, it doesnt know where it is
for the purpose of performance. In order words, the aircraft wont stop the crew from lining up
on 14R, which is 4000 feet shorter than 14L.

AhhhVC813, thanks for that, I changed my previous to make it clearer.

Cheers

Mutt.

[Denti]

mutt, on a boeing with autothrottle armed, you don't have to set thrust manually, that is done by the autothrottle system, even if not FADEC-equipped. The autothrottle is clever enough to set the thrust to the correct reduced or full thrust as entered in the fmc (most of the times anyway).

The only thing you have to do is pressing the TO/GA-switch which also does the position update to the runway-threshold.

[tired]

NWSRG - when the crew does the calculations, we take into account the runway length (or rahter, the charts we use take it into account) in determining how much weight we can lift. Once we've determined the actual a/c weight at take off we then make the decision on whether to use Flex or Max thrust. Providing you use the charts correctly, the runway length is factored in. (Sorry, am not going to get more technical than that, you'll just have to take my word for it. :) )

As mutt says, the FADEC doesn't know or care how much runway is in front of you, all it cares about is how much thrust it can give - and that depends on which detent we've set the thrust levers in. (And we've worked out which detent to use from the charts - geddit??!)

Incidentally, yes the aeroplane does measure it's own weight. Also, it does know which runway it's on because we load that info into the box during the preflight, but this is for the navigation part of the computer (so it knows where it started from!), not for the performance bit.

Hope that makes it a bit clearer.
t

[NWSRG]

tired,

Thanks for your answer, I think I'm with you now.

[N2]

Some interesting stuff mentioned here, but I would like to help clarify some issues regarding the Airbus autothrust system.

I will chat about the A340, but most other busses will be about the same.

First of all to understand what a "FLEX" thrust take off is we need to understand what the engine is limited to. For example the CFM56-5C4 is flat rated to 34,000lbs thrust at ISA plus 15 deg C. Meaning that this engine can produce 34,000 lbs of take off thrust up to an ambient temp of 30 deg C. Now here is the point, ambient temps above 30 deg C will force the engine to reduce take off thrust because we become EGT limited (take off EGT increases as outside temp increases), thrust being reduced to prevent us from melting the back end of the engine.

So, if our airplane is light and we have gobs of runway ahead of us, why push the engines to TOGA thrust when we can give them a bit of a break. As mentioned there are charts that the crew reference that will ultimately give them a "FLEX TEMPERATURE". This value mimics a very hot day usually greater than 40 deg C, and is entered into the flight management system which relays this info to FADEC. For lack of a better description, this fools the engines into thinking it is hotter outside than it really is resulting in a corresponding thrust reduction (provided the thrust levers are placed in the FLEX/MCT detent).

Second of all, although a smart computer, when it comes to thrust commands FADEC is a bit of a slave. Either the crew manually command it through movement of the thrust levers, or thrust signals are generated from the flight guidance portion of the FMGEC for autothrust. During a normal takeoff roll with either FLEX or TOGA selected, auto thrust is only armed, it will become active once the thrust levers are moved (manually) down to the "AUTOTHRUST ACTIVE RANGE", being from just above idle to the "CLB" detent. Movement of all thrust levers beyond the CLB detent will put you back in manual thrust which may result in a large swing in power as thrust hunts for blue doughnut position (a representation of thrust lever position on the N1 scale). In the event of an engine failure the AUTOTHRUST ACTIVE RANGE becomes enhanced to include up to the FLEX/MCT detent.

A bit of a long winded explanation, hope it is of use.

N2