320: Selecting TOGA when OAT<TFLEX<OAT+10

[Airmann]

On the 320 CFM or IAE

Does anyone here know of a rule which says that when TFLEX is less than OAT+10 degrees then TOGA should be used. E.g. OAT = 34C and performance calculation calls for Flex of 42C.

I am asking because there are a few instructors teaching this concept, while others say that there is no Airbus documentation implementing it and is just personal preference.

[Goldenrivett]

Hi Airmann,
is just personal preference.

I think it must be just personal, because our OPS manual says any small reduction from TOGA is beneficial :- "Even small reductions in take-off thrust can produce significant economic benefits. It is XXX airline policy to use this method whenever possible."

(CFM engines)

[Amadis of Gaul]

Our company procedure is that when flex temp is within 10deg of OAT, flex takeoff is not permitted. It's in our AOM Vol 2 very explicitly.

[Airmann]

Yea it's not in our company SOPs but I guess some prefer it. The only logic behind implementing the procedure is in order to ensure that in conditions where the temperature is rapidly increasing, fast moving front or early morning then there is a situation where you may end up with a flex lower than OAT when lining up for take off . However I believe that the aircraft can detect this situation and triggers an ECAM warning which calls for the pilot to select TOGA

[vilas]

Airmann and goldenrivett
There were in service events where OAT went above TFLEX leading to disengagement of flex take off mode, MCT thrust during take off and thrust levers not set message on ECAM. So airlines put some margin to prevent it. However as explained in operational liaison meeting 2012 Securing flexible take off FADEC logic was modified as follows:


The FLEX takeoff mode remains engaged when OAT is higher than TFLEX in order to cover small OAT variations during takeoff.

The FADEC will provide the maximum thrust available for actual conditions instead of reverting to MCT thrust (previous FADEC) up to TFLEX+10°C.

[ahramin]

As stated above, not an issue if your FADECs are current. Remember, the first degrees of FLEX are the most beneficial.

[mr.brembo]

By our SOP we use highest FLEX. But sometimes, when stop margin is less than 50 m. we decrease Flex by 2 or 3 degrees to provide aditional stop margin (we are all people). But it is not recomendation of Airbus

[vilas]

mr.brembo
It is not a recommendation because ASDA is calculated without the use of reversers so there is always some margin. But companies can have their own policies.

[Goldenrivett]

Hi mr.brembo,
when stop margin is less than 50 m. we decrease Flex by 2 or 3 degrees to provide aditional stop margin (we are all people)

That <50m stop margin would be available only if the actual airfield temperature was the same as your FLEX temperature.
Since your real OAT is much cooler, then for any given V1 IAS your TAS (and hence ground speed) is lower. So your real stop margin is much greater than displayed.

See Old Smokey's reply below.
http://www.pprune.org/tech-log/283705-performance-additional-stop-margin-rto-due-tas-effect.html