Loss of performance after takeoff

[skelsey 103]

Hello,

 

At BAV we've been receiving a number of reports from our pilots about situations where the aircraft fails to accelerate in the initial climb. The typical pattern involves a normal takeoff followed by a sluggish/non-existent acceleration leading to stall warning activation if prompt action to lower the nose is not initiated, often requiring a zero or negative rate of climb to maintain flying speed.

 

We have data showing occurrences across all models -- A318 through A321, and more detailed narrative of individual incidents if required. At first glance I can't see any particular trend in the incidents -- at first my suspicion was that it may be icing-related but more recent reports in good weather conditions would seem to debunk that. But I wondered if this was an issue that has been noted in testing or, indeed, whether there are any particular parameters that it would be helpful for us to log in order to nail the issue down?

 

Many thanks,

 

Simon

[The Dude 6537]

Hi Simon,

 

during initial testing, more than two years ago, we also came across this behaviour once in an while. Lift/drag models for an "out of clean" configuration were slightly tweaked and the beta team did not experience them anymore. With P3D the icing logic can be responsible for too much drag (will be corrected with SP4) but as you also said it can happen with good weather.

 

Parameters that can influence this behaviour are: turbulence (ASN setting above 40), weight, flex, changing wind, different accel alt from the reduction alt, AP on or off, rotation at lift off.  

 

I am most interested in cases where the reduction altitude is different from the acceleration altitude and the AP is on. When we have a "real" problem it would be most noticable here so maybe you could ask your pilots what NADP they flew and if they flew manual or not.

 

What the support guys see the most is a misshandling by the user that causes this problem. Reduction and acceleration altitude are the same when you do not change them yourself. As soon as the user sees LVR CLB they put the TL in the CL detent without thinking, this is not how it is intentent by Airbus. LVR CLB is shown when passing the reduction altitude and is a "reminder" to put the TL in the CL detent. It is not a command. The pilot should look at his energy state and decide when to activate climb thrust. If it is turbulent or the speed is very close to Vls or the nose attitude is still too high you should wait with reducing thrust. Where the reduction altitude is the same as the accel altitude you first wait for the AP or oneself to lower the nose and when you see a positive speed trend you can put the TL in the CL detent. This is how it is done IRL and by doing it this way in the sim you will most probably never get into any trouble. Not doing it this way can leave you on the negative side of the curve and this causes the problems.

 

Let us know with which kind of NADP and AP on/off it happens the most.

 

Thanks!

 

 

 

 

[Emi 5161]

Hi Simon,

 

adding to Franks post above, I have seen this happen with the IAE model if you use flex tempratures greater than 69°C.

Now there are some airlines using these in real life, however they have a higher engine rating than ours. With our engine rating this won't work out and will result in not having enough power to climb after liftoff.

If I'm not mistaken BA only flies the IAEs, so this would be a possibility.

 

If we could get some screenshots of such issues additionally to the Information requested by Frank this would be very helpfull, too.

As Frank said, to date we have seen about every possible pilot mistake, from extended spoilers, to hopelessy overweight aircraft (sometimes even in excess of 100t!).

[skelsey 103]

Thanks both, that is very helpful information. I will pass this on and see if we can get the data requested and some screenshots for you. I'll go through our existing reports as well and pull out the relevant ones.

 

As you say, BA are all IAE (with the exception of the 318), so it could indeed be linked to high flex temps.

 

Thank you!

[tup61 185]

Interesting info about reduction and accelleration attitudes and how to handle the throttle, Frank! Thanks, didn't know that! I indeed thought it was a command and always went to CL ASAP. I always have reduction and acceleration the same (don't even know how to change it and based on what...) but from now on I will only go to CL when circumstances are right. I will also Google for the difference between reduction and acceleration and how and when to change it wo what.  

 

EDIT

Think I found a good definition here:

Acceleration Height is when the nose is to be lowered to allow the aircraft to accelerate. When the aircraft starts accelerating is when the flight crew will retract flaps as per the schedule.

Thrust Reduction Height is when the autothrottle will decrease the engine power to the preselected climb thrust; thereby reducing engine wear and tear.  Both may occur simultaneously or at differing heights above ground level.  Both can be configured in the CDU.

 

I could 't yet find how to figure out the appropriate altitudes for specific airports. Should be on the charts, I suppose...?

[skelsey 103]

2 hours ago, J van E said:

 

I could 't yet find how to figure out the appropriate altitudes for specific airports. Should be on the charts, I suppose...?

It depends on a couple of things -- the type of noise abatement procedure in use (which should normally be on the charts -- usually in the textual data) which could be NADP1 or NADP2 (in which case that's probably all the chart will say) or there might be a local procedure that will be spelt out more directly.

 

However, acceleration height in particular is also performance-related. Note that the heights specified in the NADPs are minimum heights -- airlines may have standard thrust reduction/acceleration heights for each procedure that are higher than those specified where no other restriction exists, or the acceleration height may vary depending on the performance numbers generated for a particular flight on a particular day (obviously the acceleration phase results in a reduced rate of climb, which could be terrain critical at some airfields. For instance, acceleration altitudes at Innsbruck are typically in excess of 9000 or 11000ft depending on the departure being used). Modern airline EFB software will calculate a minimum acceleration height, but we don't really have such software available to us in FS -- in which case standard heights are usually sufficient unless operating out of a particularly unusual airfield (see the Innsbruck example above).

 

1000ft thrust reduction/1000ft acceleration (aal -- remember the Airbus MCDU thrust reduction and acceleration entries are altitudes (amsl) so you'd need to add the airfield elevation on) is a typical NADP2 profile which will work in most places, whilst 1000ft/3000ft or 1500ft/3000ft might be a typical NADP1 profile.

[CAS22 26]

Impressing topic,from emi and frank...i thought that all simmers know that they're not forced to comply immediately  to cl detent,in some situations(low speed at low altitude,weather surprising conditions etc...)

As former military pilot,we sometimes take over the system when it's needed.

Now,i think some simmers facing stall,or other issues as mentioned earlier due to the lack of understanding the operational method of using the throttle levels or stages,when standered circumstances aren't met at the take off or other stages in the flight.

 

[tup61 185]

Thanks for the interesting information, Simon! Much appreciated.

[Andy320 77]

I have experienced this many times in my early days flying the buses, at first I didn't have a clue what the cause was, I then later found out by luck. I noticed sometimes when I'm in a rush I tend to enter the flap and trim setting in the performance page before I've even loaded my aircraft, so my guess is, is that I will have a flex temp calculated for an empty plane (68 degree was my last one). Nowadays I enter the take off flap in the performance page after the first engine has started (normally while the second engine is spooling up to save time) to prevent any miscalculations in weight. This is something I haven't sat down and tested but I do remember the above abnormal start up sequence before this shallow climb gradient issue began. And just to add I fly only the IAE engines with an occasion pound flight with the A318 CFM once or twice a month and I haven't experienced this problem ever since I changed the sequence in which I enter the flap setting data. I hope this helps.

[LLL33 5]

On 6. Mai 2016 at 03:58, skelsey sagte:

The typical pattern involves a normal takeoff followed by a sluggish/non-existent acceleration leading to stall warning activation if prompt action to lower the nose is not initiated, often requiring a zero or negative rate of climb to maintain flying speed.

 

There are three possible reasons, I can think of:

 

- too high rate of rotation, which leads to a speed behind the power curve. The rotation speed is below V2, but with a max of 3 degrees per second it leads to a speed of around v2 + 10.

 

- f.e.  the target pitch of a 321 at max T/O weight is below 15 degrees ! Don't follow the flight director before reaching v2 + 10. SRS mode is not valid before you reach that speed  !

 

- v2 is the absolute minimum speed. If you have gusts, you need to increase your speeds (v1,vr,v2). If you fly flex in RL, you fly increased speeds. Flying v2 at high flex gives you a very low climb rate. If you go beyond that for a few seconds, you are behind the power curve, and there is no chance to recover.

 

Summary: slow rotation to at least v2 + 10 - know your target pitch for your weight and flex combi. Follow FD after reaching v2 + 10 (never earlier).