Installation of automated weather observing systems at non-airport, heliport locations shall place the sensors in the vicinity of the takeoff and landing area, and where helicopter operations will not induce transient sensor performance (e.g., rotor downwash and blowing dust causing spurious wind and visibility observations). However, no installation shall penetrate the approach and departure surfaces defined in FAA Handbook 8260.3, TERPS, or the surfaces defined in AC 150/5390-2, Heliport Design. In choosing a location, consideration will be given to both Visual Flight Rules and Instrument Flight Rules approach and departure paths and hover/taxi operations. Testing has shown no significant effect on sensors located as close as 100 feet (30 meters) from a medium weight helicopter. Another prime concern is the need to locate the sensors so as to avoid, to the maximum extent possible, conditions (sheltering and other local influences) which may result in non-representative weather observations. This may be a particular problem for heliports located in urban areas and on rooftops. The sensors, except the pressure sensors, should be located no more than 700 feet (213 meters) from the edge of the takeoff and landing area. The pressure sensor is not constrained to be at any specific location on the heliport, except to be free of rotor-induced or other pressure variations. The other sensors should be clustered for ease of installation and maintenance, but problems with non- representative sensor data or other factors may necessitate a separated location of a sensor(s).
Specific criteria for the siting of individual sensors follows (siting at airports refers to Chapter 2):
Same as for siting at airports, except the height above or below MSL shall be determined for the heliport takeoff and landing area.
Cloud height, visibility, wind, temperature/dew point, precipitation, lightning detection (thunderstorm) sensors shall be sited as indicated in paragraphs 4.3.1 through 4.3.6.
The cloud height sensor location is the same as for siting at airports, except the height is with respect to the heliport takeoff and landing area.
The visibility sensor location is the same as for siting at airports, except the height is with respect to the takeoff and landing area. To reduce the influence of dust due to rotorwash on the reported visibility, the visibility sensor should not be sited in a location which is downwind (considering the prevailing wind direction) from the takeoff and landing area.
The wind sensor will be oriented with respect to true north. The system software will be used to make required adjustments to magnetic north. The sensor will be mounted 20-33 feet (6-10 meters) above the heliport takeoff and landing area. If side mounting on a tower is necessary, a boom will be used to permit installation of the sensor a minimum of 3 feet (1 meter) laterally from the tower. Side mounting is to be utilized only if top mounting is not practicable and the tower is of open design to allow for free air flow.
The wind sensor should be located to the side of the preferred approach and departure track should be away from the sheltering influence of buildings or large trees.
The wind sensor on a building or other elevated structure should be located at least 20 feet (6 meters) above the highest structure to minimize the Bernoulli effect. Rooftop size may require siting of the wind sensor elsewhere to preclude penetration of an obstacle identification surface(s). In these situations, siting on an adjacent building may be a viable or even preferred option. It should be noted that many buildings are constructed to the maximum height that would not constitute a hazard to air navigation. Therefore, the above described siting may not be acceptable from an obstruction evaluation standpoint. In these cases, alternatives such as siting on an adjacent building may be necessary.
The temperature and dew point sensor location is the same as for siting at airports, except the height is with respect to the heliport takeoff and landing area.
The precipitation sensor location is the same as for siting at airports, except the height is with respect to the heliport takeoff and landing area.
The lightning detection (thunderstorm) sensor location is the same as for siting at airports.
When an automated weather observing system is to be sited at an airport which has, or is planned to include a heliport, a site should be chosen which will provide service to both runway and heliport users. The following options, in priority order, will be considered under such circumstances:
If siting in accordance with the applicable airport siting criteria (Chapter 3) would also comply with the criteria of paragraph 4.1, the system will be sited in accordance with the applicable airport siting criteria.
If siting in compliance with Option 1 is not appropriate, consideration will be given to an alternate location if such a location would enhance the representativeness of the data at the heliport without degrading the representativeness of the data at the primary airport runway. If such an alternate site is selected, a deviation will be processed in accordance with the directives of the responsible agency.
If siting in compliance with Option 1 or 2 is not possible, the system will be sited in accordance
with Chapter 3, or paragraph 4.1, taking into consideration such factors as volume of fixed-wing versus helicopter traffic. If siting in conformance with paragraph 4.1 is more appropriate, a deviation to use the non-airport siting will be processed in accordance with the directives of the responsible agency.
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