Enhanced flight vision system
An Enhanced flight vision system (EFVS, sometimes EVS) is a system for imaging the external world from an aircraft, and to provide an image in which objects can be better detected. In other words, an EFVS is a system to provide and image which is better than unaided human vision. An EFVS includes sensors (one or many) such as a color camera, infrared camera or radar, and typically a display for the pilot, which can be a head-up display or head-down display. An EFVS may be combined with a synthetic vision system to create a combined vision system. [1]
Functionality
The main purpose of an EVS is to permit takeoff, landing and taxiing in poor visibility conditions, where landing would not be safe otherwise. An EVS is certified for landing by the FAA only if it is combined with a HUD, in which case it is called an EFVS. [2]
The criterion for landing is known as decision height. ICAO defines Decision Height as "a specified altitude or height in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established." When a pilot in approaching the ground, they must see a visual reference to continue the approach. Visual references are for example the runway markings ("zebra stripes"), runway lighting, and approach lights (see [runway]] for more examples). If the pilot cannot see such a reference in the decision height, they must abort the landing, and then circle for a second approach or land elsewhere.
Above the decision height, the pilot uses mostly the aircraft displays. Below decision height, the pilot must look outside to identify visual references. In this stage the pilot alternates between looking at displays and looking out the window. This switching can be avoided if a HUD is installed to display information to the pilot while also looking out.
Decision Height
The decision height minimum is defined by ICAO for the following distinct categories. [3]
- Category I
- Category II
- Category III
- Category IIIa
- Category IIIb
- Category IIIc
Alternatives to EVS-assisted landing
Instrument landing system
An Instrument landing system, or ILS, relies on radio signals to allow operation in any weather. For an ILS landing to be allowed, the system must be installed on the ground, and a suitably equipped aircraft and appropriately qualified crew are required. Not all airports and runways are suitable for ILS installation, because of terrain conditions (hills in the way of the signal, non-straight landing slope).
GPS-assisted landing
While the GPS has a very high inherent precision, the reliability is not high enough for landing. GPS signals may be intentionally jammed, or lose integrity. In such cases, it may take the GPS receiver a few seconds to detect the malfunction, which is too long for critical flight stages. GPS can be used to lower the decision height below the unaided threshold, down to cat I decision height minima, but not lower.