“CLEARED FOR TAKE OFF”- when a pilot hears these words from the Air Traffic Controller, he knows it is time for him to bring the highest order of focus he can for one of the most critical phases of flight , that is ,take off.
Need for Take off Segments
One of the significant emergencies that an aircraft can face is an engine failure on take off , it means that the engine is no longer providing the necessary thrust needed.In order for the aircraft to be safe from any emergencies that may occur , it is necessary that it achieves the minimum climb gradients and clears its surrounding area and obstacles with sufficient altitude. Hence the take off segments help in achieving the above requirements.
Understanding a few terms
Before we look at the different segments of climb, it is necessary to understand a few terms related to the take off segments as it will help us understand the post better.
SCREEN HEIGHT:The take off part of the flight is the distance from where the brakes are released to the point at which the aircraft reaches a defined height.This defined height is known as screen height.It is usually 35 ft (for class A aircraft) on a dry runway and if the runway is wet it can reduce down to 15 ft.
TAKE OFF SAFETY SPEED (V2): V2 is the target speed the aircraft should attain prior to or before reaching the screen height. The reason it is called take off safety speed is because it should be attained with one engine inoperative and avoid the aircraft from stalling or the pilots loosing control of the aircraft.
NOTE:WE WILL DISCUSS THE DIFFERENT AIRCRAFT SPEEDS IN A SEPARATE POST.
CLIMB GRADIENT:The ratio of change in height (altitude gained), during a portion of a climb, to the horizontal distance traversed in the same time period.It is expressed as a percentage. It also refers to the angle at which the aircraft climbs.For an aircraft to climb , thrust has to balance drag and a part of the weight as well . Hence we require excess thrust to give us the climb gradient or angle of climb .
Climb gradient =Excess Thrust *100/Weight
The take off climb segments start from the screen height that is 35 ft above the take off surface and end at 1500 ft above the take off surface and are divided into 4 segments.
- The first segment starts when the aircraft reaches the screen height, that is 35 ft .
- The aircraft keeps climbing at the take off safety speed, that is V2 speed, until the gear is retracted.
- The objective of this segment is to expedite the climb and to make sure there is reduction in drag .
- There are two ways to reduce drag in this scenario , retracting the flaps or the landing gear.
- Since retracting the flaps very close to the ground is dangerous, we choose the option of retracting the gear .
- The first segment ends as soon as the landing gear is retracted .
- The second segment commences from the gear retraction point and the aircraft still has to maintain the take off safety speed (V2).
- The next important step is to retract flaps so that we can start accelerating the aircraft .
- However , companies have a set altitude from which the flap retraction can start for example , 400 ft AAL (above aerodrome level).
- As the gear is already retracted in the previous segment, the main source of drag is removed.
- This makes it easier for the aircraft to climb at a higher climb gradient than segment 1. The climb gradient should not be less than 2.4%.
- The main objective of the second segment is to clear the aircraft from the surrounding obstacles by maintaining the necessary climb gradient .
- The second segment concludes at 400 ft AGL or the height decided by the company from where flap retraction can commence .
- The third segment begins from 400 ft or flap retraction altitude set by the company .
- The main objective of this segment is to accelerate the aircraft so that the flaps can be retracted step by step .
- The reason we accelerate while retracting the flaps is because the stall speed will increase when we retract flaps .
- Hence as the aircraft accelerates from take off safety speed (V2) to a higher speed that is minimum drag speed or best angle of climb speed so that the aircraft doesn’t get close to stall speed .
- Once the flaps are retracted , we can set the thrust levers to maximum continuous thrust (MCT) from take off thrust .
- In the Airbus 320, take off thrust can only be used continuously for a period of 05 mins on both engines and in case of an engine failure , it can be used continuously for 10 mins.
- The segment however ends once the thrust lever are set to MCT and flaps are retracted .
- The fourth segment starts once the trust levers are set to MCT and flaps are retracted.
- The climb gradient for the last stage should not be less that 1.2%.
- In the fourth segment, the airplane is climbed to above 1500 ft AAL (above aerodrome level) where the take off flight path ends.
- In case of an engine failure , the pilot can make a decision to land at the departure aerodrome or go on further to an alternate airfield.
FACT FOR THE WEEK: This week we talk about the worlds largest passenger carrier aircraft , the Airbus A380. It is a double decker behemoth with four engines and has a cabin that can be occupied by more than 500 people .It first took flight in 2005 and was first delivered to Singapore Airlines in 2007.The Airbus A380 is also sometimes referred to as the SuperJumbo.
In 2019, it was however announced by Airbus that it would stop the production of the aircraft and the ones already in production will be delivered by 2021.The reason behind this was majority of the airlines preferring the new Airbus A350 in comparison to the A380 . The SuperJumbo will be definitely missed.
This is it for this weeks post. I hope you had a good read and got to learn something new as well . If you did , please don’t forget to likely share it with fellow aviators and aviation enthusiasts. Please feel free to comment for any questions or recommendations on the blog or further topics .Until next week , stay safe and stay healthy .