Photograph — JetRequest.com, CC BY-SA 3.0, via Wikimedia Commons
When a pilot warns of "moderate turbulence," they are using a formal turbulence reporting scale set by the Federal Aviation Administration (FAA). This four-level scale defines light, moderate, severe, and extreme turbulence in terms that pilots and dispatchers recognize and respond to over the airline's radio frequency.
The FAA’s turbulence scale defines moderate turbulence as strong airflow that can make passengers feel a definite strain against their seatbelts. Unsecured items may dislodge, and walking or food service becomes difficult. Nevertheless, the FAA emphasizes that, even in moderate turbulence, the aircraft remains in positive control at all times under the flight crew's command.
Here is a look at what each level of turbulence means within the FAA system, and what makes each one truly stand out from an aviation standpoint.
Turbulence Levels Explained
The FAA's four-level turbulence scale includes distinctions that define each level's effect on aircraft control, as well as the impact on passengers in the cabin:
Light Turbulence: Identified by the FAA as turbulence that causes the aircraft to move slightly back and forth or shake. Light turbulence is considered weak but may momentarily cause slight, erratic changes in altitude and/or attitude. The FAA guidance lists consequences such as a slight strain against seatbelts, and it may be difficult to walk or serve food.
Moderate Turbulence: Counted as turbulence similar to light turbulence but of greater intensity. It causes more notable changes in altitude and attitude, but the aircraft remains in positive control at all times. The FAA describes potential effects for passengers as a definite strain against seatbelts, unsecured objects being dislodged, and walking or food service becoming difficult. It is crucial to note that the FAA language says the aircraft remains in positive control throughout even during moderate turbulence.
Severe Turbulence: Identified as turbulence that causes large, abrupt changes in altitude and/or attitude and may cause large variations in indicated airspeed. Severe turbulence usually forces occupants to be violently thrown to a certain degree straddling their seatbelts. At this level, unsecured objects are tossed about and food service and cabin movement become impossible. The FAA warns that severe turbulence may stress the aircraft's structure and control systems.
Extreme Turbulence: The least frequently encountered variety of turbulence, extreme turbulence is identified as turbulence that can make the aircraft practically impossible to control, and may cause structural damage.
Pilots Report Turbulence Using the FAA Scale
The FAA publishes guidance urging pilots to report turbulence intensity using the standard language of light, moderate, severe, or extreme in Pilot Reports (PIREPs). The FAA explicitly states that pilots encountering "Clear Air Turbulence (CAT) conditions are urgently requested to report time, location, and intensity to the FAA facility with which they are maintaining radio contact. Pilots are requested to report time, location, and intensity to the FAA facility with which they are in communication.
Additionally, FAA weather guidance includes the key cabin consequences as part of each turbulence intensity description. This makes the passenger experience a formal reporting element that other pilots, dispatchers, and air traffic controllers are expected to use and understand.
Although the FAA does not set specific overrides or actions for each turbulence level, operational practices may cause airlines to restrict or stop food and drink service, ask passengers to remain seated and secure, and report details to the nearest air traffic control facility for dissemination to other pilots and aircraft in the area.
