Autoland is an aircraft system that can guide a plane to the runway and touch down with little or no outside visual reference. It matters most in dense fog, heavy rain, or very low cloud, when pilots may not see the runway until the final seconds. Instead of relying only on the pilot's view, the aircraft follows precise electronic guidance toward the runway centerline and glide path.
This makes landings safer and more reliable when visibility is poor.
Key Facts
- The localizer gives left and right guidance to keep the aircraft aligned with the runway centerline.
- The glideslope gives vertical guidance, commonly about 3 degrees below horizontal.
- Radio altimeters measure height above the ground by timing radio waves reflected from the surface.
- Distance from height on a 3 degree glide path is approximately distance = height / tan(3 degrees).
- Radio wave distance can be found with d = ct / 2, where c is wave speed and t is round trip time.
- CAT I, CAT II, and CAT III approaches allow progressively lower decision heights and lower visibility requirements.
Vocabulary
- Autoland
- Autoland is an automatic flight system that controls an aircraft through final approach, flare, touchdown, and sometimes rollout.
- Instrument Landing System
- An Instrument Landing System, or ILS, uses ground transmitters to send precise horizontal and vertical guidance signals to an aircraft.
- Localizer
- A localizer is the ILS signal that guides an aircraft left or right so it stays lined up with the runway.
- Glideslope
- A glideslope is the ILS signal that guides an aircraft up or down along the correct descent angle to the runway.
- Radio Altimeter
- A radio altimeter measures an aircraft's height above the ground directly below it using reflected radio waves.
Common Mistakes to Avoid
- Thinking autoland means the pilots do nothing is wrong because pilots monitor the system, set it up, verify its performance, and can take over if needed.
- Confusing GPS with ILS is wrong because a classic autoland normally follows ILS localizer and glideslope signals, not just satellite position.
- Using barometric altitude for flare height is wrong because the flare depends on radio altitude above the runway surface, not height above sea level.
- Assuming CAT III means zero risk is wrong because the aircraft, runway equipment, crew training, wind limits, and visibility rules must all meet strict requirements.
Practice Questions
- 1 A jet is on a 3 degree glideslope at a height of 300 m above the runway. Using distance = height / tan(3 degrees), how far from the touchdown zone is it?
- 2 A radio altimeter sends a pulse that returns after 1.0 microseconds. Using c = 3.0 x 10^8 m/s and d = ct / 2, what height above the ground does it measure?
- 3 Explain why an autoland system needs both ILS guidance and radio altimeter data during the final seconds before touchdown.