How Pilots Land Blind
Landing an aircraft requires a delicate balance of navigating, managing the descent of an aircraft and maneuvering at low speeds while still maintaining a safe level of lift to stay in the air. Add radio communications, weather, and air traffic to the equation and it quickly becomes a complex operation.
Civil aviation is broken down into two sets of operating rules, VFR or visual flight rules and IFR or instrument flight rules.
Visual flight rules allow an aircraft to fly solely by reference to outside visual cues, such as the horizon, nearby buildings and terrain features for navigation and orientation. Aircraft separation is also maintained visually. Visual flight rules require VMC or visual meteorological conditions.
Commercial airliners generally operate under instrument flight rules. Instrument flight rules are required in order to operate in weather conditions below visual meteorological conditions or instrument meteorological conditions. They’re also tracked and directed by air traffic control along their routes, relying on them to maintain safe separation from other traffic within controlled airspace.
The landing phase of an airliner can be broken down into 5 segments – Arrival, Initial approach, intermediate approach, final approach, and missed approach.
The final approach is the last leg before a successful landing. It can begin either from a final approach fix, an inbound vector or a procedure turn made at the end of the intermediate segment. It typically begins at a distance of 5–10 nautical miles from the runway threshold. If the pilot fails to indefinity the runway a missed approach or “go around” is initiated. This allows the pilot to safely navigate from the missed approach point to a point where they can attempt another approach or continue to another airport.
The final approach of an aircraft can be executed in several ways. If visual meteorological conditions exist and the pilot accepts it, air traffic control may direct a visual final approach.
Instrument approaches come in two primary types, non-precision, and precision. One of the most commonly used types of instrument approaches in landing is a precision approach system known as Instrument Landing System or ILS.
The first port of ILS is known as its localizer. The antennas left of the runway centerline emit an ILS radio signal with a 90Hz tone, while those on the right modulate a 150Hz tone.
The second component of ILS is known as its glide slope. This system operates similar to the localizer, except in the vertical plane and on a separate radio channel that is matched to the localizer’s channel.
The decision height is the altitude in which the pilot must forego ILS guidance and identify a visual reference to the runaway.
Category I ILS approaches are the most common type and are available to all ILS capable aircraft, including small single-engine planes.
Category II and III ILS approaches are where the critical use of aircraft automation comes into play. Known as autoland, this is done because of the low decision heights.
The autopilot systems on modern airliners are sophisticated, heavily used systems that can automatically adjust flight control surfaces in order to maintain altitude, turning maneuvers, headings, navigation points, and approaches.
In a triple redundancy system, if one of the autopilot computers requests a control input that diverges from the other two, it gets voted out of the autopilot system.
In a double redundancy system, if the two computers diverge in control input, the smaller input is used and then the autopilot system takes itself of line, triggering a fault.
The ILS ground system for both CAT II and CAT III approaches are constantly monitored for fault and must be able to switch to back-up generators quickly if power is lost.
As the aircraft enters the final approach it is configured for landing. Lift modifying devices such as flaps, slats are spoilers are extended.
For an autoland procedure, two pilots are required.
If visual contact is made with the runway, the autopilot will begin to throttle back power and begin a pitch-up maneuver known as a flare, in order to reduce the energy of the aircraft.
As the autoland proceeds after touchdown, automatic braking is deployed to slow the aircraft down.
The most hazardous part of an autoland approach is during the flare into roll-out transition as it offers a very little margin of time for responding to a failure.
The future of navigation aids suitable for automated landing lies in Ground-Based Augmentation System or GBAS. GBAS combines GPS with fixed ground-based reference stations to achieve positional accuracies in the sub-meter ranges.
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