Weather Deviation Procedures

A weather deviation is a departure from the cleared route to keep an aircraft away from a thunderstorm and the hail, turbulence, icing and windshear around it. It is everyday airline flying in convective weather: a line of cumulonimbus, such as those of the intertropical convergence zone, can push every aircraft on an airway or an oceanic track off its route at once.

Deviations need procedures because separation is planned along the cleared route. Under radar the controller sees a deviating aircraft and moves other traffic out of its way. Over the ocean, separation rests largely on position reports and data link, and an aircraft that leaves its track without a clearance can fly into the traffic on the next one. ICAO's PANS-ATM (Doc 4444) therefore sets out a weather deviation procedure for oceanic airspace, which the FAA publishes in its AIP, while the FAA AIM, the manufacturers and the operators' manuals give the distances to keep from storms.

Why deviations need procedures

Oceanic tracks are close by the standards of a storm. Lateral separation between them can be 50 NM for aircraft approved for RNP 10, 30 NM with RNP 4, ADS-C and CPDLC, and 23 NM for aircraft with RNP 4 or RNP 2 and the required data link performance. The North Atlantic organized tracks reserved for such aircraft are 23 NM apart between FL340 and FL400, so there a 20 NM deviation takes an aircraft most of the way to the next track. Vertically, RVSM puts aircraft 1,000 ft apart between FL290 and FL410. A level offset smaller than that keeps a deviating aircraft away from the flight levels assigned to everyone else.

Three different procedures move an aircraft off its centerline in oceanic airspace, and exam questions often mix them up:

Requesting a deviation in radar airspace

In radar airspace a deviation is a request like any other, but its content matters. The FAA AIM (7-1-12) asks pilots to state a heading or a number of degrees, the direction of the deviation and the approximate number of miles, for example "requesting deviation 20 degrees left for weather", and as far as possible where the detour will begin and where the route will be resumed. Once the deviation is approved, navigation within the lateral limits given by ATC is at the pilot's discretion, but the assigned altitude is kept: a change of altitude is a separate request. Requests are best made as far in advance as possible, because the controller has to assess the traffic and may need to coordinate with other sectors.

If an immediate turn is needed for the safety of the flight, the pilot in command may deviate at once, under 14 CFR 91.3(b) in the United States, and must tell ATC as soon as possible. ICAO's Annex 2 follows the same principle: the rules of the air may be departed from only when absolutely necessary in the interests of safety, and the pilot in command has final authority over the disposition of the aircraft.

Deviations begin at the planning stage, with TAFs, SIGMETs, significant weather charts, radar imagery and extra fuel for the detours. Embedded cumulonimbus, reported as EMBD in a SIGMET or on a significant weather chart, is hidden in layer cloud, so the onboard weather radar becomes essential. Datalinked weather such as FIS-B NEXRAD mosaic imagery shows where the weather was, possibly 15 to 20 minutes older than the display indicates, so the AIM accepts it for strategic route selection around an area of storms but never as the sole means of tactical maneuvering between cells.

Oceanic weather deviation procedure

The ICAO procedure, reproduced in ENR 7.3 of the FAA AIP and in chapter 10 of NAT Doc 007, starts with communication. The crew contact ATC by CPDLC or voice, as early as possible. A rapid response may be obtained by stating WEATHER DEVIATION REQUIRED, which asks for priority on the frequency and for the ATC reply, or by sending a CPDLC lateral deviation request; when necessary the call opens with the urgency signal PAN PAN, preferably spoken three times, or a CPDLC urgency message. They request clearance to deviate, giving the extent of the deviation, and tell ATC when the deviation is no longer required or has been completed and the aircraft is back on its cleared route.

If the controller can apply separation, the deviation is cleared. If there is conflicting traffic and the controller cannot establish separation, the controller says so, gives the conflicting traffic and asks the pilot's intentions; the FAA's phraseology is "UNABLE (requested deviation), TRAFFIC IS (call sign, position, altitude, direction), SAY INTENTIONS". The crew then either comply with the clearance or state their intentions and apply the procedure for flight without a clearance.

If a revised clearance cannot be obtained, a clearance is still to be obtained at the earliest possible time, and until then the crew:

• deviate away from an organized track or route system, if possible;
• broadcast the aircraft's identification, level, position and intentions at suitable intervals on the frequency in use and on 121.5 MHz, with the air-to-air frequency 123.45 MHz as a backup;
• watch for conflicting traffic visually and by reference to ACAS, if fitted, and adjust the flight path if another aircraft may be at or near the same level;
• turn on all exterior lights, within the aircraft's operating limitations;
• remain at the level assigned by ATC for a deviation of less than 5 NM from the originally cleared track;
• for a deviation of 5 NM or more, start a level change of 300 ft when about 5 NM from track, as in the table below;
• if cleared to deviate a set distance and then refused a further deviation, apply the 300 ft change before going beyond the cleared distance;
• when returning, be back at the assigned level before reaching a point 5 NM from the centerline;
• keep trying to contact ATC or, if contact was established, keep ATC advised of intentions.

On an east-west track this works out the same in both directions: an aircraft deviating to the north descends and one deviating to the south climbs. The pilot's judgment determines the sequence of actions, and ATC gives all possible assistance. The 5 NM threshold dates from Amendment 9 to PANS-ATM, applicable since 5 November 2020; older texts and exam material give 10 NM. Crews apply the version in the current AIP and, on the North Atlantic, NAT Doc 007 (see oceanic and North Atlantic operations).

Oceanic contingency procedure

The oceanic contingency procedure covers the other cases in which an aircraft cannot continue in accordance with its clearance and cannot first obtain a revised one, such as a diversion across the traffic flow, a loss of the required navigation capability or a pressurization failure. The aircraft leaves the cleared route by turning at least 30° to the right or left to establish a parallel, same-direction track offset by 5 NM, on the side where other traffic is less likely. If possible it keeps its level until established on the offset. It then establishes a vertical offset of 500 ft from the levels normally used, 1,000 ft above FL410, either at its level or after descending below FL290, the option suited to a parallel track system or a one-way traffic flow. At FL410 itself the offset is 500 ft, to FL405 or FL415. The crew keep the transponder on and squawk 7700 when able, use MAYDAY or PAN PAN as appropriate, broadcast on 121.5 MHz and advise ATC as soon as practicable. The 45° turn and 15 NM offset of older texts no longer apply, and a deviation around weather follows the weather procedure instead.

Strategic lateral offsets

The strategic lateral offset procedure (SLOP) is approved in the North Atlantic, the Pacific and other oceanic regions. Accurate navigation puts every aircraft exactly on the centerline, so an aircraft that deviates in level meets others exactly there; SLOP spreads them out. Offsets are always to the right, up to 2 NM, flown with the FMS offset function and chosen at random. In the North Atlantic, aircraft that can program them may choose any offset in steps of 0.1 NM, where older texts allowed only the centerline, 1 NM or 2 NM. An offset also relieves wake turbulence, and two crews can agree their offsets on 123.45 MHz.

Distances from thunderstorms

No single rule applies everywhere. EASA sets no figure in regulation, and operators publish their minima in the operations manual, normally from manufacturer guidance. The figures most often quoted are these:

Two severe or intense cells less than 40 NM apart therefore leave no gap to fly through, and the AIM adds that no path between strong echoes 20 to 30 NM apart or less can be considered free of severe turbulence. Hail can fall from the anvil into clear air several miles from the cell, and the threat is greater on the downwind side, which is why a deviation goes upwind of a storm whenever there is a choice. The visual appearance of a storm says nothing reliable about the turbulence inside it.

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Using weather radar

Weather radar sees precipitation, not cloud or turbulence. Wet hail gives the strongest return, while the dry upper part of a storm and its anvil, made of ice, may give almost none. The tilt keeps the beam on the wet, reflective part of the storm, below the freezing level, without overscanning the tops or painting ground clutter. Airbus gives a rule of thumb for the height of a cell top relative to the aircraft, about 100 ft per NM of range per degree of tilt: an echo that disappears at 40 NM with 1° of down-tilt has its top about 4,000 ft below. The gain normally stays in AUTO or CAL and is changed only to evaluate a cell, reduced to find the strongest core or increased at high altitude, where cloud is less reflective, then reset.

The areas to avoid by a wide margin are the steepest color gradients, where the zones crowd together, and the hooks, fingers, U-shapes and scalloped edges that suggest hail. Heavy rain attenuates the beam, so a cell behind another paints weaker than it is, and a black shadow behind a red return must be treated as potentially very active. A deviation is planned on a long range and flown on a short one, and never into a shadow or an area the beam has not scanned; Airbus wants the decision made while still 40 NM from a large storm. The HOLD function freezes the picture; comparing it with the live image a little later shows how the cells are moving relative to the aircraft, a help in choosing the side to deviate. Weak returns at cruise level above strong returns lower down mark a very turbulent area and can mean high concentrations of ice crystals, a threat to engines and air data probes even where the display shows little.

Communication failure during a deviation

The oceanic weather deviation procedure is written for the case where no clearance can be obtained, including when ATC cannot be reached at all. The crew deviate as safety requires, broadcast on 121.5 MHz, or 123.45 MHz as a backup, keep the lookout and the lights, apply the 300 ft change from 5 NM off track and keep trying to obtain a clearance. On the North Atlantic every aircraft is required to monitor 121.5 MHz and asked to monitor 123.45 MHz as well when possible, so other crews hear the broadcasts and can relay. CPDLC, satellite voice and HF offer further channels.

If two-way radio has failed altogether, the crew set 7600 and, once clear of the weather, return to the cleared route at the assigned level, which the weather procedure has them regain before 5 NM from the centerline. They then follow the regional radio communication failure procedure; on the North Atlantic, after a loss of communications inside the region, the current NAT Doc 007 has the crew maintain the cleared route, flight level and speed to the oceanic exit point, changing them earlier only when the pilot in command judges it necessary for safety.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.