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IAMSAR Search Patterns

Search patterns

  • It may be advisable for vessels, especially when searching for a person in the water with either an expanding square search (SS) or a sector search (VS), to use dead reckoning (DR) navigation rather than more accurate navigational methods.

  • DR navigation will minimize pattern distortion relative to the search object since it will automatically account for the currents affecting the search object’s drift during the search. For both vessels and aircraft, if a smoke float or other highly visible, expendable object is available, it should be deployed at a datum and the pattern should be performed relative to it.

  • Precise search pattern navigation using high-precision methods such as global satellite navigation systems will produce good patterns relative to the ocean bottom, but not relative to the drifting search object. This could allow the search object to drift out of the search area before the search facility arrives in that vicinity.

Expanding square search (SS) -

  • Most effective when the location of the search object is known within relatively close limits.

  • The commence search point is always the datum position.

  • Often appropriate for vessels or small boats to use when searching for persons in the water or other search objects with little or no leeway.

  • Due to the small area involved, this procedure must not be used simultaneously by multiple aircraft at similar altitudes or by multiple vessels.

  • Accurate navigation is required; the first leg is usually oriented directly into the wind to minimize navigational errors.

  • It is difficult for fixed-wing aircraft to fly legs close to the datum if S is less than 2 NM.

Sector search (VS) -

  • Most effective when the position of the search object is accurately known and the search area is small.

  • Used to search a circular area centered on a datum point.

  • Due to the small area involved, this procedure must not be used simultaneously by multiple aircraft at similar altitudes or by multiple vessels.

  • An aircraft and a vessel may be used together to perform independent sector searches of the same area.

  • A suitable marker (for example, a smoke float or a radio beacon) may be dropped at the datum position and used as a reference or navigational aid marking the center of the pattern.

  • For aircraft, the search pattern radius is usually between 5 NM and 20 NM.

  • For vessels, the search pattern radius is usually between 2 NM and 5 NM, and each turn is 120°, normally turned to starboard.

Track line search (TS) -

  • Normally used when an aircraft or vessel has disappeared without a trace along a known route.

  • Often used as initial search effort due to ease of planning and implementation.

  • Consists of a rapid and reasonably thorough search along the intended route of the distressed craft.

  • The search may be along one side of the track line and return in the opposite direction on the other side (TSR).

  • The search may be along the intended track and once on each side, then search facility continues on its way and does not return (TSN).

  • Aircraft are frequently used for TS due to their high speed.

  • Aircraft search height is usually 300 m to 600 m (1,000 ft to 3,000 ft) during daylight or 600 m to 900 m (2,000 ft to 3,000 ft) at night.

Parallel track search (PS) -

  • Used to search a large area when survivor location is uncertain.

  • Most effective over water or flat terrain.

  • Usually used when a large search area must be divided into sub-areas for assignment to individual search facilities on-scene at the same time.

  • The commence search point is in one corner of the sub-area, one-half track space inside the rectangle from each of the two sides forming the corner.

  • Search legs are parallel to each other and to the long sides of the sub-area.

  • Multiple vessels may use-

    • Parallel track search: for use by two ships.

  • Parallel track search: for use by three ships.

  • Parallel track search: for use by four ships.

  • Parallel track search: for use by five or more ships.

Contour search (OS) -

  • Used around mountains and in valleys when sharp changes in elevation make other patterns not practical.

  • Search is started from the highest peak and goes from top to bottom with a new search altitude for each circuit.

  • Search altitude intervals maybe 150 m to 300 m (500 ft to 1,000 ft).

  • The aircraft may make a descending orbit away from the mountain before resuming the contour search at the lower altitude.

  • The aircraft may spiral downwards around the mountain at a low but approximately constant rate of descent when there is not enough room to make a circuit opposite to the direction of search.

  • If the mountain cannot be circled, successive sweeps at the same altitude intervals as listed above should be flown along its side.

  • Valleys are searched in circles, moving the center of the circuit one track spacing after each completed circuit.

Coordinated vessel–aircraft search pattern -

  • Normally used only if there is an OSC present to give direction to and provide communications with the participating craft.

  • Creeping line search, coordinated (CSC) is often used.

  • The aircraft does most of the searching, while the ship steams along a course at a speed as directed by the OSC so that the aircraft can use it as a navigational checkpoint.

  • The aircraft, as it passes over the ship, can easily make corrections to stay on track of its search pattern.

  • Gives a higher probability of detection than can normally be attained by an aircraft searching alone.

  • Ship speed varies according to the speed of the aircraft and the size of the pattern. The relationship among the speed of the surface facility, the aircraft’s speed, the track spacing, and the length of the search legs is defined by the following equation:

Vs = (S × Va)/(L + S)


Vs is the speed of the surface facility in knots,

S is the track spacing in nautical miles,

Va is the aircraft’s true airspeed (TAS) in knots,

L is the length of the aircraft’s search leg in nautical miles.

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