Why most of the airports around the world prefer to have a 3° approach procedure to the runway?
Most passengers don’t really know or bother about the angle at which an aircraft approaches the runway from above. So, the answer to that query is a 3°approach path. It means, if we draw a straight line from the final approach altitude to the touchdown point of the runway, we will get a 3° angle. The straight line is popularly known as the Glideslope.
In aviation, to make things easier, we express the angle in terms of feet per nautical mile. A 3° approach is equivalent to 318 ft/nm, which means an airplane has to lose its altitude by 318 ft, for every 1 nautical mile it travels forward. Now the question that arises here, why is the 3° approach the most popular at airports around the world ?
- Reason 1: Comfortable rate of Descent: – A commercial jet airliner generally approaches at 140-150 knots (260-277 km/h) towards the runway on final approach. In a 3° approach path, such a fast airplane needs to descend at 700-800 ft/min to maintain the angle. It has been found that, such a rate of descent is easy on the body for passengers.
- Reason 2: Good angle for Obstacle clearance: – It has been found that a 3° approach angle gives a good clearance from nearby obstacles (hills, buildings, monuments, etc) that may be present near the approach end of the runway of most of the airports.
- Reason 3: Enable the cockpit crew to perform a smoother touchdown: – An airplane doesn’t hit the runway surface at 800 ft/min as that would be very uncomfortable, the pilots raise the nose of the aircraft a couple of degrees upwards at around 30 ft above the runway, this reduces the rate of descent significantly and enables the aircraft to touchdown at 120 ft/min or less. Since, the 3° approaches have a shallow rate of descent, it enables the pilots to just raise the nose slightly upwards to arrest the rate of descent and enables a smoother touchdown.
Is a higher approach angle used at any airports? Well, the answer is YES, various airports have approach angles slightly steeper than 3°. However, the London City Airport stands out as it has an astonishing angle of 5.5° towards the runway. This has been done in accordance with the noise abatement procedures and tall buildings near the airport. The approach is so steep, that an airplane approaching even at 135 knots (250 km/h), would require to descend at nearly 1300 ft/min to maintain the glide. The largest aircraft that can land here is an Airbus A318, that too the manufacturer had to specially configure the aircraft for such steep approaches.
A Bombardier Dash 8-200, performing a steep approach into London City Airport (EGLC/LCY).