TAKE-OFF REQUIREMENTS AND THRUST Flashcards
What factors have to be taken into accounct when considering take-off requirements?
- TOFL
- Climb requirements
- Brake Energy limits/Tyre speed limits
- Structural limits
ACN/PCN
Obstacle clearances
Define TOFL
The greater of
- ASD
- ASG
- TOD x 1,15
Define the term “Regulated Take-off weight”
The most restrictive of all relevant weight limitations
What two parts can Climb requirements be divided into?
- Climb limit mass: Air gradient that doesn’t take obstacles into account
- Obstacle limit mass: Ground gradient
What effect does flaps have on FLTOM and climb limit mass?
More flaps increases FLTOM but reduces climb limit mass
What are the obstacle clearance requirements for Take-off?
Obstacles have to be cleared by:
- 35 ft vertically or by
- 90 m + (0,125 x D)
How big is the domain widths for horizontal obstacle clearances?
- Turns less than 15 deg:
Up to 300 m in VMC by day or good nav accuarcy in IMC/Night
Up to 600 m if nav accuracy is not good enough - Turns more than 15 deg:
600 m in VMC by day
900 m in IMC or by night
Turn limitations up to 1’500 ft?
No turns up to 50 ft
Less than 15 deg up to 400 ft
Less than 25 deg up between 400 ft - 1’500 ft
What is “Reference zero”?
35 ft above runway is reference zero and that’s where the 1 st climb segment starts
Describe the 4 climb segments
- Starts at 35 ft. Ends when gear is up
Speed is V2(+10)
Gradient must be positive - Starts when gear is up and ends at 400 ft
Speed is V2(+10)
Gradient must be min 2,4% - Starts at 400 ft and ends when flaps are up
Speed is accelerating to Venr
Gradient is 0% - Starts when flaps are up and ends at 1’500 ft
In the take-off flight path, what are the gross gradients reduced by?
0,8% for 2-engines
0,9% for 3-engines
1,0% for 4-engines
Calculate climb gradient from the following:
Aircraft must be at 4’500 ft by 15 nm
What’s the required gradient?
Horizontal dist : 15 nm = 91’200 ft
Vertical gain: 4’500 ft
4’500/91’200 x 100 = 4,9%
Calculate the following:
Required gradient is 4%
TAS = 110, GS = 100
10’336/100 x 4 = 413 fpm
In 1 min @ 100 kts the a/c will travel 1,7 nm = 10’336 ft (1nm=6080ft)
10’336 ft is 100% so therefore 4% becomes 413 fpm
Describe the effect of flaps on take-off
Flaps increases CLmax
Flaps decreases the TODR
Flaps decreases the climb gradient
Describe the forces “Thrust” and “Power” when engines are spooled up just before brake release
There’s a lot of thrust but TAS is zero so there’s no power
Define “Thrust”
What happens to thrust with increase in speed?
Thurst = Mass x acceleration
Thrust reduces with speed on modern high by-pass engines because Ram effect is insufficient to compensate for intake momentum drag. 75% of the thrust is produced by the Fan. The 25% going through the core of the engine is the only air that could benefit from Ram effect so therefore the decrease in thrust
On low by-pass engines thrust is almost the same with increased speed since almost all air is going through the core
What is Ram effect and intake momentum drag?
Ram effect:
As the forward speed increases the air at the intake is compressed leading to a larger mass per unit volume. Thus improved mass flow tends to increase thrust with speed
Intake momentum drag:
Air entering the front of the engine is slowed down and loses momentum before it is again accelerated. This is called intake momentum drag and it decreases overall thrust with increase in speed.
Alternatively
When an aircraft starts moving the velocity of the air in the front of the engine increases while the air exiting the rear remains constant. This reduces acceleration and therefore thurst. At a certain speed the Ram effect increases the mass entering the engine and compensates for the intake momentum drag
On a High by-pass engine the Ram recovery effect is not very great as most of the air goes around the core and is not benefitted by Ram effect
What happens to Thrust with higher altitude?
With altitude thrust decreases due to resuced density which reduces mass flow
What happens to Thrust with temperature?
Jet thrust is practically limited by one of two things.
The TEMP limit and the RPM limit.
On hot days the air entering the engine is already hotter than normal so the TEMP limit is reached before the RPM limit.
If the outside air is cooler, RPM can be increased before hitting the TEMP limit.
Thus a reduction in outside air temperature allowing an increase in RPM causes an increase in thrust and vice versa.
If the outside air is very cold then RPM limit comes before the TEMP limit.
As long as the maximum thrust is regulated by the RPM limit it will be almost constant, independent of temperature.
Thus at high temperatures thrust varies with temperature but at low temperatures (usually below about ISA +15), the thrust is constant and the engine is said to be flat rated.
Define the term “Flat rated”
Above ISA+15 the thrust reduces as temp increases
Below ISA+15 the thrust doesn’t change with altitude
If A340 and 330 are at same weight which will higher at the screen?
The A330 will be higher. This is because if the 330 suffers an engine failure it will
lose 50% of its thrust and the 340 will only lose 25%. Therefore the 330 engines will need greater excess thrust then is required to maintain the required climb gradients. So with both engines operating it will have better performance.
