Alternate Aerodrome Planning - ATPL 070 — Operational Procedures

The minimum time route

Before any alternate is chosen, the route itself is. The minimum time route is the route that gives the shortest flight time from departure to destination while adhering to all ATC and airspace restrictions. Both halves of that definition earn their keep. Shortest flight time is not shortest distance: with a jet stream on the tail, a track that looks like a detour on the chart can beat the great circle by half an hour, which is why long-range routes bend with the day's winds.

And the route must remain legal while it hunts the wind: danger areas, closed airspace, and ATC routing requirements all constrain the search. A computed track through an active restricted area is not a minimum time route, however fast; it is not a permissible route at all. The oceanic track systems of the next module are this idea made institutional, whole families of tracks re-drawn twice a day around the wind.

The take-off alternate

The flight-preparation lesson taught the trigger, and it belongs to the rule itself: a take-off alternate shall be selected and specified in the operational flight plan when the meteorological conditions at the departure aerodrome are below the operator's established aerodrome landing minima for that operation, or when it would be impossible to return to the departure aerodrome for other reasons. The purpose is written into the same rule: a safe landing must be possible in an abnormal or emergency situation just after take-off, which is why the comparison runs against landing minima while the take-off itself may be perfectly legal.

The rule also prices the choice. The take-off alternate shall lie within a distance that minimises the risk of exposure to potential abnormal or emergency situations, and the operator shall consider at least the same four considerations the flight-preparation lesson weighed:

Weather — the actual and forecast meteorological conditions at the candidate.

Infrastructure — the availability and quality of the aerodrome's infrastructure.

Capability with a failure — the aircraft's navigation and landing capabilities in abnormal or emergency conditions, taking the redundancy of critical systems into account.

Approvals held — ETOPS, LVO and the like, which decide what the crippled aeroplane may legally fly at the other end.

How far the alternate may lie is what the flight-preparation lesson deferred to this module, and the acceptable means of compliance prices it in flying time. The take-off alternate should not be farther from the departure aerodrome than:

Two-engined aeroplanes: 1 hour of flight time at the one-engine-inoperative cruising speed from the AFM, in ISA and still-air conditions, using the actual take-off mass.

ETOPS-approved twins: the approved ETOPS diversion time, subject to any MEL restriction, up to a maximum of 2 hours at the one-engine-inoperative cruising speed, same conditions.

Three- or four-engined aeroplanes: 2 hours of flight time at the all-engines-operating cruising speed, same conditions.

SET-IMC operations: for single-engined turbines approved under Part-SPA, 30 minutes of flying time at the normal cruising speed in still air, based on the actual take-off mass.

When the AFM of a multi-engined aeroplane contains no one-engine-inoperative cruising speed, the calculation should use the speed achieved with the remaining engine or engines set at maximum continuous power. The classic traps live in the speeds, not the hours: the twin's hour is flown on one engine, the three- and four-engined aeroplane's 2 hours are flown with every engine running, and the ETOPS figure is a ceiling that the approved diversion time may stretch toward, not an entitlement. ICAO Annex 6 draws the same picture with its own accent: two engines get the one-engine-inoperative hour, three or more get the all-engines 2 hours, and an EDTO operator whose distances offer no alternate within those bands may take the first available alternate within its approved maximum diversion time. Annex 6 adds the selection gate this lesson returns to below: the available information shall indicate conditions at or above the operating minima at the estimated time of use.

One destination alternate or two

For destinations the rule works in safe-landing options rather than in alternates. At the planning stage of every IFR flight the operator shall select and specify aerodromes so that two safe-landing options are available when the flight reaches its destination. The guidance reads the arithmetic out loud: the usual pair is a runway at the destination plus a runway at a destination alternate, but two landing runways at the destination itself can satisfy the rule, provided no single event and no one weather deterioration could eliminate both options at once.

The acceptable means of compliance turns the options into counts. For each IFR flight the operator should select at least one destination alternate aerodrome. Two should be selected when the destination cannot meet the weather safety margins and planning minima of the acceptable means, in other words when the destination is not weather-permissible in the flight-preparation lesson's vocabulary, or when no meteorological information is available for it at all. Annex 6 states the same two triggers.

And the same acceptable means allows none. Outside the isolated-aerodrome case of the next section, a flight may plan without any destination alternate only when both conditions hold: the planned duration from take-off to landing does not exceed 6 hours, relaxed after an in-flight re-planning to a remaining flying time of not more than 4 hours; and the destination has two separate runways usable, with the weather from 1 hour before to 1 hour after the expected arrival showing a ceiling of at least 2,000 ft (600 m) or the circling height plus 500 ft (150 m), whichever is greater, and ground visibility of at least 5 km. Separate runways means what Annex 6 spells out: runways configured so that the closure of one does not close the other.

The isolated aerodrome and the point of no return

Some destinations simply have no alternate within sensible reach, and the rules give them their own machinery instead of pretending otherwise. The acceptable means draws the boundary in fuel: a destination should be treated as an isolated aerodrome when the alternate fuel plus final reserve needed to reach the nearest adequate destination alternate exceeds, for a turbine aeroplane, the fuel to fly for 2 hours at normal cruise consumption above the destination including final reserve, or, for a piston aeroplane, the fuel for 45 minutes plus 15 per cent of the planned cruising time, including final reserve, or for 2 hours, whichever is less. The guidance dissects the turbine block: those 2 hours contain the 30-minute final reserve and roughly 90 minutes of holding overhead.

The rule then hardens around the exception. Using an isolated aerodrome as a destination requires the prior approval of the competent authority, and for each such flight a point of no return is determined, computed by a flight-planning system and entered in the operational flight plan under the acceptable means. Before that point the flight can still divert to a fuel en-route alternate; the flight shall not be continued past the point of no return unless a current assessment of the meteorological conditions, traffic and other operational conditions indicates that a safe landing can be made at the destination at the estimated time of use. The loading arithmetic behind it, two routings priced through the same point and the greater sum carried, is the flight-planning course's predetermined-point machinery applied to the island case.

Choosing the aerodrome

What makes any candidate acceptable is judged in the window the flight-preparation lesson built, from 1 hour before to 1 hour after the estimated time of arrival at that aerodrome, and the acceptable means splits the family across its two yardsticks. A take-off alternate or a destination should show weather at or above the applicable landing minima: the RVR or visibility of the operating minima, plus, for a type A or circling operation, a ceiling at or above the MDH. A destination alternate, a fuel en-route alternate or an isolated destination should show weather at or above the planning minima, the deliberately stiffer figures of the flight-preparation lesson's table, because these aerodromes are insurance and must still be open if the day degrades. For the take-off alternate and the isolated destination, any limitations tied to one-engine-inoperative operation should be taken into account on top.

The guidance adds the practical test of usability: an instrument approach operation is considered usable for the planning minima only when the approach facilities are available, the aircraft is equipped to perform such an approach, the flight crew is accordingly trained, and the runway is available for landing. An immaculate ILS on a closed runway secures nothing.

Navigation failure gets its own wall. For each IFR flight the operator shall ensure that sufficient means remain to navigate to and land at the destination or at any destination alternate if the capability for the intended approach is lost. When the operation relies on PBN, the acceptable means turns this into a selection rule: an aerodrome should be chosen as destination alternate only if an instrument approach that does not rely on GNSS is available there or at the destination. Operational credits such as EFVS or SA CAT I follow the same logic: the alternate pairing should include an approach that does not lean on the same credit. The guidance bounds the wall precisely, and the bounds are quotable: it constrains destination alternates only, a take-off or en-route alternate with GNSS-only approaches may be planned without restriction, and a GNSS-only destination may be flown without any alternate when the no-alternate conditions of the previous section are met.

The en-route alternate rounds off the family. It is the member a flight diverts to while en route, and three things qualify one: it must be adequate like every alternate; when it serves as a fuel en-route alternate it should clear the planning minima in its own arrival window; and it must be reachable on the fuel actually carried, which the guidance ties to the failure cases, since reaching an en-route alternate with an engine failed or the cabin unpressurised may require additional fuel from the fuel-scheme rules. Annex 6 adds the operational family tie: en-route alternates are what extended diversion time operations by twins are required to select, and that regime is the next lesson entire.

The minimum time route

The take-off alternate

Check you understood

One destination alternate or two

The isolated aerodrome and the point of no return

Choosing the aerodrome