ASRS Directline

  Issue Number 2 : October 1991

Between a Rock and a Hard Place Weather Deviations

by Ed Arri

Each year both pilots and controllers are confronted with weather-related problems that have a significant impact on the safety of flight, and on the air traffic system as a whole. Pilots want to deviate around build-ups they see and/or observe on their airborne weather radar as "red cells." In the face of weather mandated route or altitude changes, the controller must maintain standard separation from other aircraft. Pilots frequently blame controllers for not understanding their need to deviate. Controllers, on the other hand, believe pilots have little idea of what is involved in granting such requests and the subsequent impact on other traffic.

Different Jobs, Different Viewpoints

The air traffic control system is designed to handle a large number of aircraft within a highly standardized route structure. Whenever weather becomes a factor, workload for both the pilot and controller are greatly increased. Since weather has little regard for the standardized route structure, the air traffic control system at that particular time and location demands non-standard remedies to reduce the negative impact on all aircraft. Controllers will, if they are able, approve deviations around the "red cells" for passenger comfort, and more importantly, for safety. Most of the time these deviations can be approved with minimal impact on the system; however, there are times when even slight deviations can create enormous problems for the controller. Adding to the control problem is the movement of the storm. It generally doesn't stay in one place long enough for the controller to work out some sort of routine with other sectors/positions.

The pilot has relatively few options when it comes to avoiding severe weather. The forces of nature can be extremely nasty at times. The instinct for survival tells the pilot that the weather ahead is bad stuff, and must absolutely, positively, be avoided. When ATC approval for deviation is denied, solutions and alternatives must be communicated and worked out by both the controller and pilot. Of course, all of this is taking place while the aircraft continues to head toward the problem.

Controller's Perspective

Many reports received at ASRS from controllers indicate that weather deviations have been responsible for a loss of separation between aircraft, and have frequently resulted in the controller being charged with an operational error. What the pilot wants to do does not always conform to ATC handbook requirements, and occasionally is contrary to good ATC practices. Allowing pilots to deviate from standard routes greatly diminishes the controller's ability to effectively provide positive separation between aircraft--the separation provided by the standard route structure suddenly does not apply. Aircraft can easily enter the adjacent controller's airspace without coordination due to the sheer volume of traffic and distractions. There is little time to coordinate new headings and routes with other ATC facilities because of frequency and interphone congestion.

A controller may also be unable to stop other traffic from entering his/her airspace right away due to coordination requirements. Traffic flow can't be turned on and off like a faucet.

One controller involved in an operational error reported that

--a loss of separation occurred.

The more aircraft that are deviating, the more problems the controller must contend with; the controller's ability to provide positive control to all aircraft under extreme conditions may be compromised. It's like having a tiger by the tail and you're afraid to let go.

The controller does not have authorization to use less-than-standard-separation, except in emergencies. When confronted with situations that limit their ability to provide positive control to all IFR aircraft, controllers encounter an increased risk of operational error. Operational errors are taken very seriously by the controller and the FAA. They may result in the controller being "off the boards" from two days to two weeks, and sometimes longer while the investigation and recertification process is conducted.

Pilot's Perspective

The number one priority for the pilot is safety. A request to deviate around weather is based on known factors that tell the flight crew some sort of action is necessary to remain clear of the adverse weather conditions ahead--for the well-being of the aircraft and its occupants.

Pilots may believe that controllers do not appreciate the risks that confront pilots in heavy weather. One pilot who was not allowed to deviate around a thunderstorm system reported:

Many pilots believe there should be enough flexibility in the system to handle these adverse situations. They feel that if coordination with the next controller is necessary to allow an aircraft to deviate around weather, then the controller should go ahead and do it. The pilot does not want to play twenty questions before the deviation is finally approved. The pilot may also be reluctant to declare an emergency when the request to deviate is denied. One reporter claims that a "...request to squawk 7700 is an invitation to paperwork."

Some Examples...

Restricted Airspace

An air carrier flight on an airway wanted to deviate to the left around a large thunderstorm, but the controller was reluctant because of a nearby restricted area.

Conflicting Traffic

A Center controller had aircraft deviating around thunderstorms during moderate to heavy traffic conditions. Two aircraft on conflicting courses were unable to comply with ATC instructions due to build-ups along their route of flight. A loss of separation occurred between the aircraft.

A departing air carrier discovered thunderstorm cells on radar and requested deviations around them. The controller was unable to approve the request due to heavy departure traffic in front and behind.


The following comments indicate some typical reactions whenever requests cannot be granted by either the pilot or controllers:

Weather Emergencies

ASRS reports indicate a reluctance on the pilot's part to declare an emergency whenever the "all else" fails. In the following report, the flight crew needed to deviate around thunderstorms, but the controller could not approve the deviation since it would take the aircraft into a restricted area.

In another instance, a Center controller working aircraft with thunderstorm activity in the area approved a pilot's request to deviate, but due to the heavy concentration of aircraft and limited flexibility in the airspace, had to restrict where the aircraft could go.

Reducing The Impact

Timely communication can help the pilot avoid thunderstorms while still allowing the controller to provide separation from other traffic. Last minute requests are difficult to coordinate.


  1. Don't assume that the controller knows where all the thunderstorm activity is located. Tell him what you want and what you can do, not what you can't do, when making your request.

  2. Plan ahead--give the controller as much notice as possible so that inter/intrafacility coordination can be accomplished in a timely manner.

  3. The pilot is responsible for the operation of the aircraft and the safety of its passengers. Timely PIREPS can help the controller work with the pilot in accomplishing this by formulating a traffic plan in advance and relaying this information to other aircraft.


  1. Controllers need to minimize last minute surprises by finding out exactly what the pilot has in mind when they request clearance to deviate. Carte Blanche approvals can lead to problems.

  2. Controllers too should plan ahead. Developing a good plan for future traffic flow, and letting flight crews know in advance what's going on will go a long way toward reducing conflicts and last minute surprises.

When All Else Fails...

  1. Since the controller is not authorized to go below minimum-required separation unless an emergency is declared, and will do whatever is necessary to insure that separation loss does not occur, the final decision on the course of action rests with the pilot.

  2. Pilots are reluctant to declare an emergency. However, in certain situations, there may be no other alternative available to the pilot. FAR 91.3(b) states that: "In an in-flight emergency requiring immediate action, the pilot-in-command may deviate from any rule of this part to the extent required to meet that emergency."

  3. The Airmen's Information Manual (AIM), paragraph 441, states: "An aircraft is in at least an urgency condition the moment the pilot becomes doubtful about position, fuel, endurance, weather, or any condition that could adversely affect flight safety."

  4. Once the pilot declares an emergency, the controller can provide advisories and other services until the emergency situation no longer exists and normal radar or vertical separation can be reestablished.

Summing Up

Good planning by both the pilot and controller, an awareness of adverse weather conditions, effective communications, the willingness to endure a little paperwork, and mutual cooperation are the key elements to reducing the impact of being Between A Rock and a Hard Place.

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