Minimum Separation Distances Prescribed in Southwest Airlines' Thunderstorm Avoidance Policy

Air Safety Week, April 23, 2001

On a recent television documentary about air crashes in foul weather, some useful guidance was provided by one of the on-camera experts. His words are particularly timely for this time of the year, when the annual thunderstorm season begins. "Avoid, avoid, avoid," declared John Hammerschmidt, a member of the National Transportation Safety Board (NTSB).

But by how much should pilots avoid severe storms? The development and progression of these storms can be exceedingly dynamic, and they can catch the unwary aviator by surprise. For some carriers, Hammerschmidt's words pretty closely replicate their policy. Nevertheless, the Safety Board may be looking for a more specific policy than a generic one of avoidance. This was clearly evident in the Board's hearings in January 2000 into the fatal June 1. 1999, crash of an American Airlines [AMR] MD-82 at Little Rock, Arkansas. The airplane landed in the midst of a strong thunderstorm. (see ASW, Feb. 14, 2000).

The line of questioning during those hearings indicated that Safety Board investigators were looking for minimum separation distances from thunderstorms. The investigation is still ongoing.

Perhaps the most specific thunderstorm avoidance policy is the one used by Southwest Airlines [LUV]. As at American Airlines, Southwest pilots train to the precipitation reflectivity that is presented on their airborne weather radar, and the Southwest guidance is based on these radar echoes. Note that in warmer weather (32? F or above) pilots must maintain at least a 5 nautical mile (NM) separation from all echoes. This minimum separation distance increases to 20 NM if the airplane is not going to clear the cloud top by more than 5,000 feet. Moreover, when flying above 23,000 feet, all weather radar echoes must be avoided by at least 20 NM.

According to Mark Clayton, the carrier's safety officer, this year all Southwest pilots received a package by mail containing a booklet and an interactive CD on radar performance and turbulence avoidance. These instructional materials reinforce the carrier's thunderstorm and turbulence avoidance guidelines.

The minimum separation distances in Southwest's thunderstorm avoidance policy have two primary purposes: they not only predispose crews to stay well clear, the policies provide time for flightcrews to keep their options open. As was attributed to the late Gen. George S. Patton, "Weather's not important until it's important, and then it's too late."

'Extreme Avoidance' Thunderstorm Avoidance Guidelines

* When the temperature at flight level is 0 degrees Celsius or higher (? 32? F), avoid all echoes (on the weather radar) by 5 nautical miles.

* When the temperature at flight level is less than 0 degrees Celsius (? 32? F), avoid all echoes by 10 nautical miles.

* Avoid all echoes by 20 nautical miles if the aircraft clears the tops (of the thunderclouds, as shown on the radar) by less than 5,000 feet. All weather with radar tops above 15,000 feet may be hazardous.

* All weather with radar tops above 20,000 feet is hazardous.

* When flying above 23,000 feet, avoid all echoes by 20 nautical miles.

* Never fly near an echo with a radar top above 30,000 feet.

* If possible circumnavigate echoes by flying on the up wind side.

* Circumnavigate all echoes with steep or asymmetrical gradients.

* Assume that all magenta radar returns are severe thunderstorms.

* NEVER assume that ATC (Air Traffic Control) will provide warning of hazardous weather. (Emphasis in original)

* NEVER assume that a PIREP (Pilot Report) will provide warning of hazardous weather.

* NEVER continue flight toward a radar shadow.

* Note: Although a general or severe thunderstorm may have a well defined base, hazards often exist from the ground to the base of the convective cell, even in VMC (Visual Meteorological Conditions).

Source: Southwest Airlines, Flight Operations Manual, Weather Procedures, reproduced with permission.