Instrument Departures (SIDs and ODPs)

An instrument departure gives an IFR aircraft a protected path from the runway to the en-route structure. The procedure designer surveys the obstacles around the aerodrome and publishes a track, a climb gradient and sometimes a set of take-off minima. A pilot who follows the track and climbs at least as steeply as published is clear of obstacles; one who turns early, climbs too slowly or leaves the route is not.

ICAO's design criteria are in PANS-OPS (Doc 8168) and the United States uses TERPS, but both rest on the same geometry. The vocabulary differs: European AIPs publish standard instrument departures (SIDs) and omnidirectional departures, while the FAA divides departure procedures into SIDs and obstacle departure procedures (ODPs). In both systems the design assumes all engines operating; the engine failure case is a separate performance problem (see take-off flight path and obstacle clearance).

Standard instrument departures

A standard instrument departure (SID) is a published, coded route from the runway to a point on the en-route network, charted in the AIP according to ICAO Annexes 4 and 11. It serves obstacle clearance, noise abatement and traffic flow at once, and lets a controller clear a whole route in a few words. The ICAO designator combines a basic indicator, normally the name of the significant point where the SID ends, a validity number and a route letter: BELUS 2C at Chambéry leaves the 355° climb at BANEK, 8.0 NM from the CY DME, and turns left towards LB624.

SIDs are published as tracks to be made good, so the pilot corrects for wind; under radar vectors the pilot flies the assigned heading and ATC allows for the wind. Charts carry altitude and speed constraints at fixes, such as "MAX IAS 190 kt" and FL090 "at or below" at LB601 on the Chambéry runway 18 RNAV SID, and distinguish fly-by from flyover waypoints: BANEK is a flyover point, so the turn begins only once it has been overflown. RNAV SIDs are normally RNAV 1 or RNP 1, requiring a total system error within 1 NM for 95% of the flight time, and are loaded by name from a current navigation database and checked against the chart (see area navigation); arrivals follow the same principles in reverse (see STARs).

In France a pilot on a SID reports on first contact with approach the call sign, the SID designator and the level to which the aircraft is climbing. In the United States a SID is flown only when ATC has cleared the aircraft for it, and a pilot who does not want one writes "NO SID" in the flight plan remarks. The FAA names SIDs in words, such as SHEAD TWO DEPARTURE (RNAV).

Obstacle departure procedures

An FAA obstacle departure procedure (ODP) exists solely to clear obstacles by the least onerous route to the en-route structure. It is printed as text in the Takeoff Minimums and (Obstacle) Departure Procedures section of the Terminal Procedures Publication (TPP), or as a chart with "(OBSTACLE)" in its title, and approach charts for the airport carry a "T" symbol pointing to it. An ODP may be flown without ATC naming it, unless ATC assigns a SID or radar vectors, and 14 CFR 91.175(f) requires Part 121, 125, 129 and 135 operators taking off under IFR to use a published ODP or an alternative that ATC assigns. At a non-towered airport ATC normally assigns an ODP only when it is needed to separate aircraft, but pilots may fly it for terrain and obstacle clearance.

Altitude and speed restrictions on an ODP exist for obstacle clearance and ATC cannot cancel or amend them. If ATC vectors an aircraft off an ODP, or assigns an altitude below one it publishes, the ODP is cancelled and ATC takes over terrain and obstacle clearance. A diverse vector area (DVA) lets ATC give radar vectors during an uninterrupted climb from the runway instead of an ODP or SID; it may also carry a climb gradient. A Part 91 pilot who departs IFR without an ODP, SID or DVA takes on obstacle clearance personally, and the AIM strongly encourages flying a departure procedure at night, in marginal VMC and in IMC.

Departure end of runway and the OIS

The design begins at the departure end of the runway (DER). From there an obstacle identification surface (OIS) slopes upwards along the departure: 2.5% in PANS-OPS, and in TERPS a 40:1 surface rising 152 ft per NM, the same slope, until the minimum IFR altitude or the en-route structure is reached. An obstacle that penetrates the surface forces the designer to publish a steeper gradient, higher take-off minima, a specific route, or a combination.

In PANS-OPS the minimum obstacle clearance is zero at the DER and grows by 0.8% of the distance flown. TERPS assumes the aircraft crosses the DER at least 35 ft above its elevation, climbs to 400 ft above it before turning, and then climbs at least 200 ft per NM to the minimum IFR altitude. The FAA assesses obstacles to 25 NM from the airport, or 46 NM in designated mountainous areas; if nothing penetrates the 40:1 surface within that area, the runway passes a diverse departure assessment and no ODP is needed.

The FAA divides takeoff obstacles into two kinds. Low, close-in obstacles need a steeper climb only within 1 NM of the DER and to 200 ft or less above it, so no higher minima are published, but weather of at least 300 ft and 1 NM may be needed to see and avoid them. Takeoff minimums obstacles, within 2.6 NM of the DER and higher than 200 ft above it, bring higher ceiling and visibility minima. Newer notes also give a DER crossing height that clears them.

Procedure design gradient

The procedure design gradient (PDG) is the minimum climb gradient on which the obstacle protection rests. The default is 3.3%, the 2.5% surface plus 0.8% clearance, which is about 200 ft per NM: the ICAO and FAA figures describe the same slope. A higher gradient is published only where needed, with the altitude to which it applies and the obstacle that imposes it; beyond that point, 3.3% applies again until the aircraft reaches the minimum altitude for the en-route phase. At Cannes the SID from runway 17 needs 7.5% up to 1,000 ft because of 840 ft terrain at Théoule, then 3.3%. French criteria do not publish a PDG above 15% caused by obstacles very close to the DER; the gradient is recomputed without them and their position and height are published instead.

Some gradients serve ATC rather than obstacles. Chambéry's BELUS 2C needs 6.5% up to 4,200 ft for every departure, plus an ATS gradient of 9.3% to reach FL110 at BELUS when cruising at FL110 or above; a pilot unable to meet an ATS gradient tells ATC. The FAA is phasing ATC climb gradients out of its SIDs, but any published gradient is mandatory once the procedure is part of the clearance, unless higher take-off minima are offered and the weather allows them.

A gradient becomes a rate of climb through the groundspeed. A percentage times about 60.76 gives feet per NM, and feet per NM times groundspeed divided by 60 gives feet per minute:

A headwind lowers the rate needed and a tailwind raises it. The published gradient assumes all engines: the one-engine-inoperative net flight path required by performance rules is a different calculation, and airlines publish engine failure procedures for each runway.

Straight and turning departures

A straight departure keeps the initial track within 15° of the runway centreline. When a turn of more than 15° is needed, the procedure becomes a turning departure, turning at a specified altitude or height or at a fix, and PANS-OPS provides 90 m (295 ft) of obstacle clearance in the turn. French criteria assume straight flight until at least 120 m (394 ft) above the DER, a figure that may be lowered for Categories A and B where necessary but never below 90 m (295 ft). TERPS assumes no turn below 400 ft above the DER; a turn published at a higher altitude is started at that altitude, and a turn at a fix is made at the fix. Crews must also respect published maximum speeds, since the protected turn area is drawn for them.

The FAA still has a few departures instructing "turn left (or right) as soon as practicable" to avoid obstacles straight ahead; they carry minima of at least 300 ft and 1 mile and are being phased out.

Omnidirectional (diverse) departures

An omnidirectional departure lets the aircraft set course in any direction, or within published sectors, once a set height has been reached. PANS-OPS requires a climb on the extended centreline to 120 m (394 ft) before any turn, and at least 90 m (295 ft) of obstacle clearance before turns of more than 15°. The French AIP distinguishes type A, a climb on a set magnetic track to a set altitude, and type B, a climb to a fix, usually the IAF; both assume a continued climb at the published gradient, or 3.3%, up to the minimum safe altitude. At Brest runway 25L, Categories A and B climb on the runway axis to 1,000 ft, but Categories C and D to 3,000 ft, with left turns forbidden. The FAA's equivalent is the diverse departure.

Non-standard take-off minima

Under 14 CFR 91.175(f), standard takeoff minimums for Parts 121, 125, 129 and 135 are 1 statute mile for aircraft with two engines or fewer and ½ mile for more than two; Part 91 has none, though flying at least the published minimums is wise. Where obstacles demand it, the TPP publishes non-standard takeoff minimums, usually with an alternative. At Arlington, Washington, runway 34 has standard minimums with a climb of 260 ft per NM to 800 ft, or a 1,200 ft ceiling and 2½ miles for a pilot who cannot achieve it. Some airports offer a visual climb over airport (VCOA), a visual climb above the field to a published altitude, as an alternative to a steep gradient against obstacles more than 3 statute miles from the DER; others mark a runway "NA-Obstacles", closing it to IFR departures. Operations specifications can authorise lower values, down to RVR 500 with centreline lights and RVR 300 with a HUD or other take-off guidance.

French aerodrome charts publish minimum take-off RVR per runway and aircraft category, such as 550 m for a Category C aeroplane on runway 17L at Lyon. Take-offs below 400 m RVR need a low-visibility approval under Part-SPA. In commercial air transport, when the departure weather is below the landing minima, a take-off alternate is required, within one hour at one-engine-inoperative cruise speed for a twin.

Climb via clearances

Climb via is FAA phraseology for a SID with published altitude restrictions. It clears the pilot to fly the lateral path, meet every published or assigned altitude and speed restriction, and climb to the SID's top altitude. "Climb via SID except maintain flight level one eight zero" replaces the top altitude while keeping the other restrictions. A later "climb and maintain" cancels the published altitude restrictions, but published speed restrictions remain unless ATC deletes them. After a climb via clearance the pilot reports the altitude leaving and any assigned restriction not on the chart on first contact.

Because one designator stands for a whole route with its level and speed restrictions, a short clearance carries a great deal. The French AIP calls the first-contact report of the SID and the level being climbed to a confirmation of the SID clearance: it lets the controller check that the crew is flying the procedure it was given. Under FAA rules, vectors off a SID cancel it, with its altitude and speed restrictions, unless the controller adds "expect to resume SID".

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.