Low Visibility Procedures (LVP)
Aerodrome Operating Minima
Aerodrome operating minima is, in general terms, a package of minimum criteria limiting the usability of an aerodrome for take-off and landing.
"The cloud ceiling and Runway Visual Range for take off and the Decision Height or Minimum Descent Height, Runway Visual Range and visual reference for landing which are minimum for the operation of that aircraft at that aerodrome."
Cloud Ceiling - Vertical distance from the elevation of the aerodrome to the lowest part of any cloud visible from the aerodrome which is sufficient to obscure more than half of the sky
Decision Height - The height in a Precision Approach at which a missed approach must be initiated if the required visual reference to continue has not been established.
Minimum Descent Height - The height in a non precision approach below which descent may not be made without the required visual reference.
Visual Reference - A view of the section of the runway and/or the approach area and/or their visual aids which the pilot must see in sufficient time to assess whether or not a safe landing may be made from the type of approach being conducted.
Runway Visual Range (RVR) - RVR indicates the range over which the pilot of an aircraft on the centreline of a runway can expect to see the runway surface markings, the lights delineating the runway or identifying its centreline.
Instrumented RVR (IRVR) - IRVR is an automatic and continuous display of RVR values. It is measured at 3 points alongside the runway; Touchdown, Mid-point, Stop end. The RVR is measured by dalek-looking instruments called Transmissometers. They stand opposite each other and measure the opacity of the air between them.
Human Observer RVR - This method uses a human to calculate the RVR. A qualified person stands on a platform at the side of the runway and counts the number of runway centreline lights they can see. Then, using a look-up table, they can see the distance X amount of lights corresponds to.
Aerodrome operating minima is not calculated by ATC - It is calculated by aircraft operators. They take into account the aircraft, crew, aerodrome, runway and aids when calculating AOM. Pilots are responsible for their own AOM - ATC are not to police it.
When are Low Visibility Procedures Operational?
LVPs are in force if any of the following are true:
The UK standard for reporting RVR or IRVR extends from zero to 1500m:
RVR values are to be passed to aircraft at the beginning of each approach for landing, and thereafter, whenever there is a significant change in the IRVR until the aircraft has landed. The IRVR is to be passed to aircraft before take-off.
As RVR's change rapidly, RVR isn't included in the ATIS. It will sometimes appear on METAR reports. In our virtual world, we can only pass RVR indications when they appear in the METAR report. The RVR indication is the touchdown reading, and will be in the format of R(runway)/(rvr) e.g. R33/800.
|Approach category||Decision height||Runway visual range (RVR)|
|I||200 ft (61 m) or more||"1|
|II||less than 200 ft and more than 100 ft (30 m)||"1|
|IIIa||less than 100 ft and more than 50 ft (15 m)||600 feet (180 m)|
|IIIb||less than 50 ft (15 m) or none||150 feet (46 m)|
During Low visibility procedures, the ILS signal has to be protected due to automated landings. Something called the ILS Sensitive Area (set distance either side of the runway centreline) is set up. No vehicle or aircraft should be within the ILS sensitive area from the time an arriving aircraft is within 1nm from touchdown until it has completed its landing run or departing aircraft commences take-off run until it is airborne.
This means runway holding points have to be moved further back from the runway. Final approach spacing has to be increased to accommodate the fact that aircraft will be slower vacating he runway.