Alternate Aerodromes

An alternate aerodrome, or alternate airport in FAA usage, is the place a flight can go when it cannot or should not land where it planned. ICAO defines it as an aerodrome to which an aircraft may proceed when it becomes impossible or inadvisable to proceed to or land at the aerodrome of intended landing, where the necessary services and facilities are available and the aircraft's performance requirements can be met. There are three kinds: a take-off alternate for the minutes after departure, an en-route alternate for a problem in the cruise, and a destination alternate for the arrival.

Alternates are chosen before flight, on forecasts that may be hours old by the time they are used, which is why the weather they must show is set higher than the minima for actually landing. Choosing one also commits fuel: the flight must carry enough to reach it with the final reserve intact (see fuel planning and fuel reserves). Destination alternates are filed in item 16 of the ATC flight plan and en-route alternates in item 18.

Adequate and suitable aerodromes

Two words run through every alternate rule. An adequate aerodrome is one the aeroplane can actually use: EASA defines it as an aerodrome on which the aircraft can be operated, taking account of the applicable performance requirements and runway characteristics. For extended-range planning it must also be expected to be available at the anticipated time of use, with ATS, lighting, communications, weather reporting, navigation aids, rescue and fire-fighting cover and at least one instrument approach. The FAA's adequate airport (14 CFR 121.7) is similar: it meets the landing limitations of 121.197 and the Part 139 airport standards other than those for rescue and fire-fighting.

A suitable aerodrome is an adequate aerodrome that the day's conditions also allow: weather reports and forecasts at or above the planning minima, and a runway surface on which a safe landing is expected, for the whole period in which it might be used. Since October 2022 EASA rules also use weather-permissible aerodrome for an adequate aerodrome whose reports or forecasts for the anticipated time of use are at or above the aerodrome operating minima, with runway condition reports indicating a safe landing. Adequacy is fixed by the aeroplane and the aerodrome; suitability changes with every forecast.

A usable alternate also needs the practical checks: that it will be open with ATS and lighting at the expected time, has a runway suitable for the aeroplane, and has an approach the crew and aircraft are qualified and equipped to fly, with no NOTAM withdrawing it. The departure aerodrome may be nominated as destination alternate if it passes the same tests. If the destination has only GNSS-based approaches, the alternate should offer one that does not depend on GNSS, so that a single satellite problem cannot close both.

Take-off alternates

A take-off alternate is an aerodrome at which the aeroplane could land shortly after take-off if it could not return to the departure aerodrome, for example after an engine failure when the departure weather is below the landing minima. That is exactly when one is required, under EASA rules for commercial air transport and under 14 CFR 121.617 alike.

Its distance is measured at the speed the aeroplane would actually fly after the failure:

Under the EU-OPS rule still widely examined, the speed is taken in standard conditions at the actual take-off mass, and the forecast at the take-off alternate must be at or above the applicable minima from 1 hour before to 1 hour after the estimated time of arrival there.

Destination alternates

A destination alternate is the aerodrome to which the flight may proceed if it becomes impossible or inadvisable to land at the destination. There is no fixed distance limit; the limit is the fuel.

An IFR commercial flight normally needs at least one. The long-standing EU-OPS exemption, still common in exam questions, allowed the destination alternate to be omitted only if the planned flight time was no more than 6 hours, the destination had two separate usable runways, and the forecast from 1 hour before to 1 hour after the ETA gave a ceiling of at least 2,000 ft or circling height plus 500 ft, whichever is greater, and a visibility of at least 5 km. The current EASA rules keep the runway and weather conditions. An isolated destination, with no alternate within reach, needs the competent authority's approval and additional fuel instead.

A second alternate is required when the destination forecast, from 1 hour before to 1 hour after the ETA, is below the applicable planning minima, or when there is no meteorological information for the destination at all. Two alternates that meet planning minima then compensate for the doubtful destination, and alternate fuel is calculated for whichever needs more.

Private IFR flights under Part-NCO have a lighter test. Since 30 October 2022 NCO.OP.140 allows the destination alternate to be omitted only when, from 1 hour before to 1 hour after the ETA, or from departure to 1 hour after the ETA if shorter, the forecast gives a ceiling at least 1,000 ft above the DH or MDH of an available instrument approach and a visibility of at least 5,000 m.

En-route alternates

An en-route alternate (ERA) is an aerodrome at which the aircraft could land after an abnormal or emergency condition in the cruise. For twin-engined aeroplanes the distance to one limits the route itself. Under EASA rules (CAT.OP.MPA.140), as under EU-OPS before them, a performance class A twin with 20 or more passenger seats, or a maximum take-off mass of 45,360 kg or more, must stay within 60 minutes' flight at one-engine-inoperative cruise speed of an adequate aerodrome unless approved for extended operations; smaller class A twins have 120 minutes.

Beyond that, ETOPS approval is needed, and the flight plan must name ETOPS en-route alternates within the approved diversion time, or the time allowed by the aircraft's MEL status if shorter. Each must be suitable, with its forecast at or above the ETOPS planning minima, for the period in which the aeroplane might need it.

The 3% ERA, or fuel ERA, is a different kind of en-route alternate, selected so that contingency fuel can be cut from 5 % to 3 % of trip fuel.

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EASA planning minima

Planning minima are the weather an alternate must be forecast to have for it to be selected, deliberately padded above the landing minima because the forecast is used hours in advance.

Since 30 October 2022, aerodrome selection for commercial air transport aeroplanes sits in CAT.OP.MPA.182 and its AMC, part of the fuel/energy scheme of Regulation (EU) 2021/1296. The AMC table applies to destination alternates, isolated destinations, fuel ERAs and en-route alternates, and adds increments to the published minima of the approach available:

Where more than one row applies, the operator may use the most favourable. An alternate served only by one non-precision approach with an MDH of 350 ft and a minimum visibility of 1,500 m therefore needs a forecast of at least 750 ft and 3,000 m. ETOPS en-route alternates are planned with increments of 200 ft and 800 m for a precision approach, and 400 ft and 1,500 m for a non-precision or circling approach. Operators publish the planning minima they apply in their operations manuals, and those govern on the line.

FAA 1-2-3 rule

The FAA rule for Part 91 IFR flights is known as the 1-2-3 rule. Under 14 CFR 91.169(b), an alternate need not be filed if the destination has a standard instrument approach procedure and the forecast, for 1 hour before to 1 hour after the ETA, shows a ceiling of at least 2,000 ft above airport elevation and a visibility of at least 3 statute miles. If the destination has no instrument approach, an alternate is always required. For helicopters the window runs from the ETA to 1 hour after, and the figures are 1,000 ft above the airport or 400 ft above the lowest applicable approach minima, whichever is higher, and 2 miles.

Both figures must hold for the whole window. A TAF giving 3 statute miles but a 1,500 ft overcast an hour before the ETA fails the ceiling test, so an alternate must be filed, and 91.167 then requires fuel to it plus 45 minutes at normal cruising speed. Part 121 has its own versions: domestic flights use the same 2,000 ft and 3 miles in 121.619, while flag operations under 121.621 may dispense with an alternate only on flights of 6 hours or less with a similar but stricter forecast.

Standard and non-standard alternate minimums

Alternate minimums, or alternate weather minimums, are the FAA's planning minima. Under 91.169(c) the forecast at the alternate must meet, at the ETA, the alternate minimums published for its approach, or if none are published the standard alternate minimums:

An airport with no instrument approach may be filed only if the forecast allows descent from the MEA, approach and landing under basic VFR. These are planning figures only. On arrival the pilot flies the published landing minimums of the approach actually used. Helicopters use a ceiling 200 ft above the approach minimum and at least 1 mile visibility.

Non-standard alternate minimums are flagged on the approach chart by a white A in a black triangle, and listed in the alternate minimums section of the Terminal Procedures Publication. At Pangborn Memorial (EAT), for example, the VOR-B requires 2,000 ft and 3 miles for a category C aeroplane rather than 800-2. An A followed by NA means the approach may not be used for alternate planning, often because weather reporting is missing or the facility is unmonitored.

Airlines work differently. Under 121.625 the alternate must meet the minimums in the carrier's operations specifications, derived in OpSpec C055 from the landing minimums: with one navigation facility, 400 ft is added to the MDA or DA and 1 statute mile to the visibility; with two facilities serving straight-in approaches to different suitable runways, 200 ft and half a mile are added to the higher of the two approaches. The 600-2 and 800-2 figures do not govern a Part 121 release.

Second alternates and Exemption 3585

Under EASA rules a second alternate follows from a destination forecast below planning minima, as above. Under Part 121, at least one additional alternate must be designated when the weather forecast for both the destination and the first alternate is marginal.

Conditional forecast groups make this common in practice. FAA guidance treats a TEMPO group below minimums within the ETA window as controlling, which would normally rule the airport out as destination or alternate. Many US carriers hold Exemption 3585, which permits a release when the conditional part of the forecast is below minimums while the prevailing forecast is at or above them, provided a second alternate is listed on the release. How PROB groups are treated is set by each carrier's operations specifications and manual.

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.