TAKE-OFF REQUIREMENTS AND THRUST

Question 1

What factors have to be taken into accounct when considering take-off requirements?

Answer 1

1. TOFL
2. Climb requirements
3. Brake Energy limits/Tyre speed limits
4. Structural limits
   ACN/PCN
   Obstacle clearances

Question 2

Define TOFL

Answer 2

The greater of

• ASD
• ASG
• TOD x 1,15

Question 3

Define the term “Regulated Take-off weight”

Answer 3

The most restrictive of all relevant weight limitations

Question 4

What two parts can Climb requirements be divided into?

Answer 4

1. Climb limit mass: Air gradient that doesn’t take obstacles into account
2. Obstacle limit mass: Ground gradient

Question 5

What effect does flaps have on FLTOM and climb limit mass?

Answer 5

More flaps increases FLTOM but reduces climb limit mass

Question 6

What are the obstacle clearance requirements for Take-off?

Answer 6

Obstacles have to be cleared by:

• 35 ft vertically or by
• 90 m + (0,125 x D)

Question 7

How big is the domain widths for horizontal obstacle clearances?

Answer 7

1. 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
2. Turns more than 15 deg:
   600 m in VMC by day
   900 m in IMC or by night

Question 8

Turn limitations up to 1’500 ft?

Answer 8

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

Question 9

What is “Reference zero”?

Answer 9

35 ft above runway is reference zero and that’s where the 1 st climb segment starts

Question 10

Describe the 4 climb segments

Answer 10

1. Starts at 35 ft. Ends when gear is up
   Speed is V2(+10)
   Gradient must be positive
2. Starts when gear is up and ends at 400 ft
   Speed is V2(+10)
   Gradient must be min 2,4%
3. Starts at 400 ft and ends when flaps are up
   Speed is accelerating to Venr
   Gradient is 0%
4. Starts when flaps are up and ends at 1’500 ft

Question 11

In the take-off flight path, what are the gross gradients reduced by?

Answer 11

0,8% for 2-engines
0,9% for 3-engines
1,0% for 4-engines

Question 12

Calculate climb gradient from the following:
Aircraft must be at 4’500 ft by 15 nm
What’s the required gradient?

Answer 12

Horizontal dist : 15 nm = 91’200 ft
Vertical gain: 4’500 ft
4’500/91’200 x 100 = 4,9%

Question 13

Calculate the following:
Required gradient is 4%
TAS = 110, GS = 100

Answer 13

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

Question 14

Describe the effect of flaps on take-off

Answer 14

Flaps increases CLmax
Flaps decreases the TODR
Flaps decreases the climb gradient

Question 15

Describe the forces “Thrust” and “Power” when engines are spooled up just before brake release

Answer 15

There’s a lot of thrust but TAS is zero so there’s no power

Question 16

Define “Thrust”

What happens to thrust with increase in speed?

Answer 16

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

Question 17

What is Ram effect and intake momentum drag?

Answer 17

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

Question 18

What happens to Thrust with higher altitude?

Answer 18

With altitude thrust decreases due to resuced density which reduces mass flow

Question 19

What happens to Thrust with temperature?

Answer 19

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.

Question 20

Define the term “Flat rated”

Answer 20

Above ISA+15 the thrust reduces as temp increases

Below ISA+15 the thrust doesn’t change with altitude

Question 21

If A340 and 330 are at same weight which will higher at the screen?

Answer 21

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.