Description

VFR over-the-top (VFR OTT) means that a portion of the flight is conducted over the top of the cloud layer (i.e. without ground reference) while in accordance with visual flight rules. The procedure is different to VFR on Top, which is applicable to IFR flights.

VFR OTT is not an ICAO procedure. Annex II: Rules of the Air sets a requirement that the surface should be in sight only for flights below 3000 ft AMSL / 1000 ft above terrain (whichever is higher) and only in Airspace classes F and G. For all other situations a horizontal (1500 m) and a vertical (300 ft) clearances from the clouds are prescribed.

Some countries, e.g. Canada (CAR 602.116) and Germany (LuftVO, para 32), however, impose additional restrictions on VFR flights, e.g. stating that these must not be conducted unless there is visual reference to the surface and allow VFR OTT if additional conditions are met. For Canada these are:

• the aircraft is operated at a vertical distance from cloud of at least 1,000 feet
• where the aircraft is operated between two cloud layers, the vertical distance between the layers is at least 5,000 feet
• flight visibility at the cruising altitude of the aircraft is at least five miles
• the weather at the aerodrome of destination is forecast to have no broken, overcast or obscured layer lower than 3,000 feet above the planned flight altitude and the ground visibility is forecast to be five miles or greater with no thunderstorms or precipitation

The flying conditions (visibility and turbulence) above cloud are generally more favourable compared to those below. It is therefore tempting to fly above the clouds even if that means losing terrain reference. However, there are some factors that need to be considered, e.g.:

• Navigation using ground features may be impossible
• The flight will have to eventually descend below the cloud in order to land. If the cloud is above the intended destination aerodrome, this may mean that the flight will either have to divert, or be prolonged.
• Additional fuel may be required, e.g. in case the flight needs to be prolonged or needs to divert to an unplanned alternate.
• If the pilot intends to use a gap between clouds, its size must be at least 3 km in diameter and it is quite possible that the available ones are smaller. 
• Clouds develop, and sometimes the process is very fast. This applies to both their horizontal size and top. A situation may develop where the cloud has risen higher than the aircraft can climb.
• Using a gap between clouds to descend below them may result in finding out that the clouds extend to the surface so flying below them under VFR is not possible.
• Trying to use a gap that does not provide the prescribed horizontal clearance from the clouds may increase the risk of collision. This clearance is intended, among other things, to provide sufficient time for "see and avoid" regarding IFR traffic exiting the cloud.
• If there is no suitable way to get below the clouds, the aircraft may need to descend in IMC. This could be particularly challenging for pilots without an IFR rating.

Related Articles

• VFR on Top
• Inadvertent VFR Flight Into IMC