Weather Avoidance Services

CHAPTER SEVEN

INSTRUMENTS FLIGHT PLANNING

705.

WEATHER AVOIDANCE SERVICES

As a result of radar advisory service information, the pilot may decide to alter his/her flight to

avoid severe weather areas. Before doing so, however, he/she must first obtain a clearance from

ATC to deviate from his/her present clearance if on an IFR Flight Plan. There are two methods

of receiving radar vectors to avoid thunderstorms and other severe weather while proceeding on

an IFR Flight Plan.

First, request permission from the controlling ATC to switch to PMSV for radar weather

advisories. If you desire to deviate around the weather, permission must be obtained from

ARTCC to fly off the airway in a specified direction and distance. In low density traffic areas,

ARTCC may approve flight off the airway for this purpose. The aircrew must return to the

ARTCC frequency immediately upon completion of the diversion.

In the second method, radar vectors around areas of severe weather may be obtained in some

situations directly from the ARTCC. This is the more common method. However, the primary

purpose of the center's radar is aircraft separation, and the controller normally uses reduced

video gain to eliminate weather from the scope. Upon request, severe weather can sometimes be

painted on the radar receiver, and the controller may give vectors to direct aircraft around it or

through the less severe sections of the storm. It should be noted that vectoring service can only

be offered when it does not interfere with the controller's primary duty of maintaining flight

separation.

706.

WEATHER CRITERIA FOR FILING

Flight plans are filed, based on:

1. The enroute, destination and alternate weather forecasts for a period of one hour prior until

one hour after ETA at your destination and the actual weather at the point of departure at the

time of clearance.

2. Meteorological conditions notwithstanding, IFR Flight Plans are filed and flown whenever

practicable as a means to reduce midair collision potential. In any case, forecast meteorological

conditions must meet the weather minima criteria (Figure 7-2) for filing IFR Flight Plans and are

based on the pilot's best judgment as to the runway that will be in use upon arrival.

IFR Flight Plans are filed for destinations at which the weather is forecast below the

appropriate minimums if a suitable alternate airfield, forecast to have at least 3000 feet ceiling

and 3 statute miles visibility, is available during the period one hour before and one hour after

the ETA.

A suitable alternate must include a published approach compatible with installed aircraft

navigational equipment, or forecast ceiling and visibility which permit descent from the

minimum enroute altitude, approach, and landing under VMC.

7-8 IN-FLIGHT WEATHER ANALYSIS