Dear friends and followers today I'll be answering another great question
which was sent in by one of my YouTube subscribers
Joe I've heard pilots talking about Flex temperature
what's that? That's a great question so let's get started
Okay first off the Flex temperature is always referred to the takeoff
performance setup for many jet powered airplanes except for Boeing
They called it the assumed or derated temperature
Okay in my example we will refer to an Airbus A320 with CFM turbofan jet engines
Now many passengers have the impression that pilots
always apply full power when performing a takeoff
But do you remember the last time you sat in an Airbus
that shortly after takeoff the engine sound
decreased a little bit as if the power would drop
Well that's due to the Flex temperature and the climb power setting
So prior to every takeoff you have to perform the takeoff calculations
taking many factors into account
First and foremost the aircraft takeoff weight
The ramp agent will hand you a load sheet
showing all the important figures you need for the calculations
including the stabilizer trim setting
and then you check the local ATIS and start filling up the module
So let's go with this example
We have the takeoff weight of 68 tons and these environmental factors
Once the calculation is finished you'll get this data
So what's all this? You have the V1, the Vr and the V2
Those are the speeds in the next topic for the next video
and the Flex temperature
Now let's use the suggested flex temperature of 58 degrees
So you type in 58 degrees on your performance page
and now the FADEC, the Full Authority Digital Engine Control unit
assumes that the outside air temperature is 58 degrees
and therefore the FADEC reduces the power purposely
to reduce the engine wear in such "expected" hot temperatures
So when applying the thrust levers to the Flex detent
the FADEC will give you a 85.3% takeoff power setting
Although you know there's some extra power left
which you can always access by applying
TOGA at one further detent on the thrust lever column
giving you maximum thrust available
The result of using a high Flex temperature
is that the speeds are higher for liftoff because the plane
won't accelerate as quickly as it could
and therefore using up more runway
resulting in a shorter stop distance
in case of rejected takeoff
So the conclusion of that is the lower the Flex temperature
so the fake outside air temperature for the FADEC
the more power will be set when applying thrust
So the closer it is to TOGA
The higher the Flex temperature
the more the engines are derated
resulting in a longer takeoff roll with higher speeds
and at the same time reducing the engine wear
So how can you as a passenger hear or feel the difference
which Flex temperature was chosen?
The best example is Lanzarote airport
the airplane is close to the maximum
takeoff weight on a very short runway
therefore the plane needs a lot of power
to accelerate as quickly as possible
to reach the rotation speed
The engine will literally roar much louder
and after liftoff and reaching the thrust reduction altitude
There's a reason why they've named it the thrust reduction altitude
where you set the takeoff thrust to climb thrust
and you'll hear and feel the power change
a reduction in noise and deceleration in the climb rate
But why do that in the first place?
Why not just set TOGA thrust for every takeoff
and go through all that hassle
First and most importantly is to increase the engine life
therefore reducing the chance of an engine problem due to technical failure
more controllability of the aircraft with lower thrust output
in case of an engine failure on the runway and just after liftoff
and obviously the fuel flow is lower using a Flex temperature
To sum up you would say the Flex temperature
is the sweet balance between the stop margin available on the runway
and a longer engine life and fuel economy
But there are cases where the use of Flex temperature is prohibited
in case the runway is contaminated with standing water
or slush, snow or ice or whatever
if the runway is very short at high altitudes
and hot outside air temperature
or performance reasons due to obstacle clearance in the departure sector
the takeoff module will then calculate a TOGA thrust setting for you
Now maybe you've experienced this
you're sitting at Boeing and as you gently lift up into the air
and once you've reached your thrust reduction altitude
suddenly more power is applied
what's going on?
Some takeoffs on the Boeing, the thrust for the takeoff is
reduced even below the climb thrust
so let's say, derated power is 75%
and the climb power is at 85%
You can feel and hear the increase of those 10%
in power at the first reduction altitude
and therefore thank you very much for your time
please subscribe to my channel
and check out my instagram account by the name @flywithcaptainjoe
and spread the word. See you next week
All the best your Captain Joe