PSS Airbus FLEX/TOGA take-off question

[martinlest2 58]

I almost invariably take off by starting pushing the throttles forward until N1 is at 50%, then engaging the FLEX option. When I get to the Thrust Reduction & Acceleration altitude and LVR CLB flashes on the instruments, I go to THRUST to CL. That always works just fine.

 

I was flying in the Chinese/Tibetan Himalayas today (great scenery!) and taking off from altitudes of up to 14000', the FLEX option just doesn't generate enough thrust to get the aircraft comfortably off the ground, even on 12000' runways. So I used the next gate, TOGA, and all was well. However, I then found that pressing the '-' key to get back to climb thrust didn't work. I get FLEX or SPEED.. and the autothrottle doesn't seem to work at those settings, the aircraft gradually loses speed. The only way to get thrust is to select TOGA again (when you are soon overspeed -  again the A/Thr seems to be ignored), or to cancel right down so there is nothing indicated on the instruments (no A/Thr) and use the throttles.

 

Is that what should happen? I've got so used to FLEX take-offs that from a TOGA I am a bit thrown by what is/isn't happening.

 

Should I be able to adjust the FLEX TO TEMP to adjust my FLEX take-offs to the departure situation, rather than use TOGA? I am not sure how effectively these things are modelled in the PS a/c...

 

Thanks,

 

Martin

[Luis Hernandez 564]

Martin, I understand your first cleared altitude should be higher than 14000, and this should be entered in the MCP, and dashes in the speed and heading values (I guess you're already aware, but just for confirmation). Also, if you're at TOGA, you're now 2 detents ahead of CL (instead of 1), so, when you get the LVR CLB message, you need to press twice the Num- key (not just once). If you press only once, you get FLEX (as expected) or MCT.

 

I do always this when departing SKBO (but it's only 8300 ft high). Try again (initial altitude a lot over 14000, double '-' key at reduction height) and let us know what happens, please.

[skelsey 765]

Hi Martin,

 

Flex is Airbus's term for assumed temperature thrust reduction. The flex temperature would be calculated prior to departure dependent on the conditions for a particular takeoff: runway length, obstacles in the climbout, takeoff weight, air temperature and density altitude etc. In the sim world, we have software such as TOPCAT which can do this for us or there is also a freeware FS Airbus takeoff performance calculator available on the Internet.

 

By nature, operations at high altitude airfields are challenging. The reduced air density means a higher TAS for a given IAS, thus resulting in an increased ground roll and takeoff distance (and likewise when landing, the higher TAS results in a much steeper/faster descent and more energy required to be dissipated by the brakes, thus leading to a longer landing distance required). In addition, the engines will produce less thrust because the reduced air density means that they will be throwing a lower mass of air out the back for a given power setting.

 

Most aircraft (including the A320) are only certified as standard for takeoff and landing up to around 9800ft and operations at airfield above this elevation require special certification and quite often special equipment, sometimes including uprated engines capable of producing more thrust or (commonly) specially-configurable pressurisation systems that don't freak out when the cabin altitude exceeds 10,000ft.

 

Therefore I would suggest use of TOGA would probably be advisable as chances are any performance tables that either you or any performance software might have installed are unlikely to contain data for operations at the sort of elevations you are describing and therefore the chances of coming to a sensible Flex solution are limited.

 

Now to the Airbus operation itself.

 

As Luis mentioned, in the A320 the autothrust only becomes active (A/THR white on the FMA) when the thrust levers are in the Climb detent (or below). Above CL (ie in FLX/MCT or TOGA) you are in manual thrust (hence the FMA indication "MAN FLX" or "MAN TOGA") and the autothrust will normally be armed (A/THR displayed in blue), ready to activate when the thrust levers are set to the climb detent.

 

As Luis mentioned, in TOGA you are obviously now two detents above Climb and therefore you will need to 'click' the thrust levers back twice in order to get in to the climb detent and allow the autothrust to activate.

 

Hope that helps.

[martinlest2 58]

 

Martin, I understand your first cleared altitude should be higher than 14000, and this should be entered in the MCP, and dashes in the speed and heading values

Yes, I always set up the MCP that way.

,..if you're at TOGA, you're now 2 detents ahead of CL (instead of 1), so, when you get the LVR CLB message, you need to press twice the Num- key

I understood that :smile: but two presses didn't bring me down to THRUST to CL. Maybe my installation has a problem. I'll try again later and double check. Thanks.

 

Yes, as I say, I was using TOGA rather than FLEX as usual and takeoff (from 14000') was fine, it was just a question of getting back to climb thrust, which didn't seem to happen, no matter how many times I clicked the - or + key. The next flight I tried setting my Flex TO Temp to 20 degrees (I understand in reality settings lower than 30 are pointless, but whatever) - and then using the FLEX gate for take-off. Worked like a charm. So perhaps I will do this in future, rather than TOGA, when I need extra thrust for whatever reason. I will test out the TOGA again though, to see if I can get climb thrust this time. If not, I'll post back exactly what happens..

 

Thanks again.