Erroneous glideslope at BIA

Monday June 16 2008

Monica Jha

BANGALORE: Pilots from two airlines (one domestic and one international) have reportedly complained about problems in landing using instrument landing system (ILS) at the Bengaluru International Airport.

The runway (27 orientation, which is being used presently) at the new airport has a 3.4 degree glideslope for an ILS approach while the international standard for the same is 3 degree. The 3 degree glideslope (the angle of descend with respect to horizontal plane) is an acceptable descent profile world over and, therefore, auto pilots are designed for this profile. At BIA, which has a glideslope of more than 3 degrees, auto pilots do not work and pilots need to resort to manual landing, that can cause hard landing at times.

A three-degree glideslope gives a descent of approximately 318 feet per nautical mile (NM) while for a 3.4-degree glideslope, the descent would be about 370 feet per mile which means an aircraft would descend at a higher speed than recommended.

When an aircraft on ILS follows a 3 degree glideslope, passengers do not feel any discomfort but a 3.4 degree glideslope may result in a steep landing causing discomfort to passengers. The operator of BIA, Bangalore International Airport Limited (BIAL), apparently made a mistake in assessing the elevation of glide path location.

On a continuous descent, an aircraft must maintain a height of 50 feet at the threshold of a runway. Due to errors in calculating the elevation this height at BIA was found to be less than 50 feet. So, BIA had to increase the glideslope to maintain a height of 50 feet at the threshold of runway. However, the glideslope for runway when used in 09 orientation is 3 degrees.

To ratify the problem, BIA would need to relocate its glide path antenna further up, an expert from the aviation industry told this website's newspaper. "But, changing the position of glide path antenna at a live airport and a live runway is not possible as landing without ILS for a few days that would be necessitated for calibrations, would be extremely difficult. BIAL can do it when they change from 27 to 09 runway in November," he added.

Source : The New Indian Express