Toga vs Flex on the 320 series

[PGA]

Just a quick perf question referring to my current type, the 320 series.

Performing a flex take off, with a flex of say, 60 degrees, while using the "mean" V1, on a 3000m runway, a typical V1 is around 140. While this drops to around 120 when you use Toga.

I see you would use Toga to get airborne quicker, obstacles, windshear etc, however, why for a given config does the V1 drop so much?

The ASDR when going from Flex to Toga genearlly drops too, i.e. if you decided to Toga the ASDR is lower, however would you guys, if you decide to use TOGA at the last minute, re-do the perf calculations and use the lower V1, or is it acceptable to use your Flex V speeds, and select TOGA at the last minute? I can see this would be an issue if your field length limited, but if not, are there any significant issues in doing this?

Thoughts and comments appreciated!

[hetfield]

kvs - factor?

[Permafrost_ATPL]

The goal of flex is to reduce engine wear while accepting the potential risk of higher speed rejects. In other words, because you have less thrust it will take longer to get to V2 after engine failure and hence the higher V1. Higher V1 means potentially more hazardous stop.

Thus when using TOGA the calculated V1 is lower (plenty of thrust to get to V2 at 35/15 feet).

It is "acceptable" to use speeds based on flex and use TOGA but it defeats the purpose of flex in the first place:)

P

[Gumby]

At a former airline of mine, it was stated they never had an engine failure using "flex" (standard, reduced, etc.). The only engine failure I have had at an airline was a TOGA T/O, we didn't need to do it but the captain (and he is the captain) wanted to use it. We got to 60 knots when the #3 blew every blade out the back.

Long story short, back to the gate, looked through the engine from one end to the other, seeing daylight all the way, and commuted home, taking 12 hours. That was our last leg of a five day trip.

I am a firm believer in flex because I like to be gentle with my tools, take care of them and they take care of you. :ok:

[OPEN DES]

The A320 is almost never field limited. So Vspeeds for a FLEX take-off are optimized/increased (improved climb in non-airbus speak) in order to maximize the FLEX temperature. i.e. a higher V2 will result in a a better climb gradient thus a higher climb limited TOW. That's why you can see very high Vspeeds on long runways.

If you use TOGA, there is no FLEX temp to be maximized. So the Airbus philosophy is to have the lowest speeds possible. (tirewear, ASDA, brakes etc). Also note that when Tmaxflex (ISA+60 for most CFM's, ISA+53 for IAE) is achieved there is no further Vspeed optimization. So in very non-limiting cases you CAN see FLEX with lower than usual Vspeeds or even 'TOGA speeds'.

You can use TOGA with FLEX speeds, no problems. (think about it: you would consider selecting TOGA when N-1).

[FatFlyer]

Hi,
my understanding of the computerised FOVE performance is that it looks for the maximum thrust reduction by using an improved climb technique (higher V2) to achieve 2nd segment and obtacle clearance gradient. The detailed results for a perf calculation give you a wide range of V1 options.

The only reason I can think of for a higher V1 with TOGA would be if you were VMCG limited and needed more rudder authority with full thrust on the live engine. Maybe the lower V1 with TOGA is the lowest safe useable V1 for TOGA ?.

I have also wondered why on some long dry runways, V1 comes out so low when there is plenty of room to stop well beyond V1

(edit. just seen previous post which is far better than my limited explanation)

[OPEN DES]

Just a small note:

On a flex take-off Vmcg will still be Vmcg for TOGA, same for Vmca. Since TOGA can always be applied......... For a flex take-off all the TOGA requirements will still have to be met.

Vmcg/a can only be reduced with a de-rate. With de-rate I mean that the thrust-rating of the engine is physically reduced (done on some Boeings).

[FatFlyer]

Thanks for info on VMCG, it would have to be a derate otherwise you could select TOGA and go off the runway which I should have realised.

[FE Hoppy]

Why would you "decide to use TOGA at the last minute"?

More info is required to analyse your numbers but the ASDR is obvious isn't it? You need to accelerate to Vgo and then decelerate to 0. All other things being equal then more thrust will get you to Vgo sooner and therefore in less distance. The deceleration will be the same for both cases.

Gumby. I've rejected twice. Both were for failure following bird ingestion. No amount of previous husbandry would have prevented them.

[Tail-take-off]

Why would you "decide to use TOGA at the last minute"?



Many reasons,for example:

Forgot to set the flex temp. (If you have never done this you must be the only person who hasn't:ok:)

Engine failure at MTOW from some where like Addis Ababa. (Its a long time between V1 & Vr & your going to climb slowly enough in TOGA).


I'm sure there are many more....

[OutOfRunWay]

why not use the better gradient with TOGA? And, please noone come here with that rubbish "harder to control laterally with full thrust".

regards.

[bluesky919]

Hi all!
Please help me ,i'm doing my training to become F/o of the AIRBUS A320/1, just only have 70 flying hrs on this type.Normally our airlines (Vietnam airlines) has take off chart to find out speed for t/o,flex temp base on the weight,config,wind.But usually on A320( QUITE OLD SERIES),when i set flex( for ex 67 degree) in to MCDU page PERF, then during take of roll,when i set Power Liver to FLEX/MCT position,the FMA appear MAN FLEX 66 ( 1 DEGREE less than the number i set in MCDU during prepairation).I don't know why ?.Thanks for reading my question,enjoy your flights.HOpe to see your answer soon

[Metro man]

Why would you "decide to use TOGA at the last minute"?

Aircraft not performing as expected. Where all the calculations corrrect (ZFW, correct runway chart used etc) ? Full power, climb away and work it out later. :ok:

[VnV2178B]

Tail-take-off,

may I ask what you would like the engines to do if you do forget to set a flex temperature?

declaration of interst here as I am looking into future automatic responses and one of the scenarios is just that.

Currently as far as I can see there is a nagging message which asks for either a temperature to be input or TOGA to be selected on the TLs.

VnV

(but of a misnomer now as I have been 'promoted' to systems stuff)

[qatarstudent]

Why would you "decide to use TOGA at the last minute"?

in case of engine failure after V1...you don't have(require more lat. trim), but it s nice to have it.

[mcdhu]

bluesky919. I Have no explanation for you, but one of our sims (CFM) does that for no apparent reason. So far the sim techs don't know why, but we take the view that as long as the PFD FMA 'Man Flex' number is less - then it is safe.

Perhaps somebody else will come up with an explanation.

Cheers,
mcdhu

[bluesky919]

mcdhu!
thanks for your concern about my question,up to now,i still haven't got the answer yet.In my opinion( just guess),maybe the FADEC give the number from PERF page to calculate for the engine performance,after calculate based on FLEX T we put in,FADEC decides that with the parameters put in,he must reduce 1 degree to have the best perf for engine.Hop to hear more explan about this.Enjoy your flight!

[OPEN DES]

w.r.t. the 1deg flextemp difference:

From my observations:

Sometimes the RTOW charts or FOVE take-off performance module will come up with a flex of Tmax flex. (ISA+60 or ISA+53 for CFM, IAE respectively). This is ofcourse corrected for elevation.......

1. It can be sometimes that there is a totally acceptable discrepancy between the elevation used by the RTOW chart/FOVE and the actual elevation when FLEX thrust is applied.
2. Also the FADEC will not round up Tmax flex to the closest whole degree I would think, but rather always round down to the lowest whole degree to NEVER exceed the Tmax flex limitation. The RTOW charts/FOVE will correct Tmax flex for elevation ofcourse but might round up at times, especially when elevation is just a few feet of a whole round 1000 ft figure (e.g. 2002 ft).

Just my thoughts......