AIM ¶ 4-1-16 — ATC Safety Alerts
AIM 4-1-16 explains ATC safety alerts: terrain/obstruction warnings, MSAW monitoring, and aircraft conflict alerts for pilot students prepping for checkrides.
In Plain English
AIM 4-1-16 describes the Safety Alert that ATC issues when, in the controller's judgment, your aircraft is in unsafe proximity to terrain, obstructions, or other aircraft. The service depends on the controller's awareness — it cannot be mandated, but you can expect it on a reasonable, intermittent basis. Once issued, the pilot decides what action to take.
There are two types:
- Terrain/Obstruction Alert — Triggered primarily by Mode C altitude reports. Example phraseology: "Cessna Three Four Juliett, check your altitude immediately..." Most en route and terminal radar facilities use an automated tool called Minimum Safe Altitude Warning (MSAW) to flag tracked Mode C aircraft predicted to descend below a safe altitude.
- Aircraft Conflict Alert — Issued when traffic not under the controller's control creates an unsafe proximity. The controller will, if feasible, give traffic position and suggest an alternate heading or altitude.
MSAW monitoring is automatically provided to IFR aircraft with operating Mode C, and VFR pilots may request it. MSAW may be inhibited near the Mexico/Canada borders due to incomplete terrain data. Safety alerts are reserved for time-critical situations; routine traffic situations use normal traffic advisories.
AIM Source Text
FAA AIM ¶ 4-1-16A safety alert will be issued to pilots of aircraft being controlled by ATC if the controller is aware the aircraft is at an altitude which, in the controller's judgment, places the aircraft in unsafe proximity to terrain, obstructions or other aircraft. The provision of this service is contingent upon the capability of the controller to have an awareness of a situation involving unsafe proximity to terrain, obstructions and uncontrolled aircraft. The issuance of a safety alert cannot be mandated, but it can be expected on a reasonable, though intermittent basis. Once the alert is issued, it is solely the pilot's prerogative to determine what course of action, if any, to take. This procedure is intended for use in time critical situations where aircraft safety is in question. Noncritical situations should be handled via the normal traffic alert procedures. Terrain or Obstruction Alert Controllers will immediately issue an alert to the pilot of an aircraft under their control when they recognize that the aircraft is at an altitude which, in their judgment, may be in an unsafe proximity to terrain/obstructions. The primary method of detecting unsafe proximity is through Mode C automatic altitude reports. Most En Route and Terminal radar facilities have an automated function which, if operating, alerts controllers when a tracked Mode C equipped aircraft under their control is below or is predicted to be below a predetermined minimum safe altitude. This function, called Minimum Safe Altitude Warning (MSAW), is designed solely as a controller aid in detecting potentially unsafe aircraft proximity to terrain/obstructions. The radar facility will, when MSAW is operating, provide MSAW monitoring for all aircraft with an operating Mode C altitude encoding transponder that are tracked by the system and are: Operating on an IFR flight plan; or Operating VFR and have requested MSAW monitoring. Due to the lack of terrain and obstacle clearance data, accurate automation databases may not be available for providing MSAW information to aircraft overflying Mexico and Canada. Air traffic facilities along the United States/Mexico/Canada borders may have MSAW computer processing inhibited where accurate terrain data is not available. Aircraft Conflict Alert. Controllers will immediately issue an alert to the pilot of an aircraft under their control if they are aware of another aircraft which is not under their control, at an altitude which, in the controller's judgment, places both aircraft in unsafe proximity to each other. With the alert, when feasible, the controller will offer the pilot the position of the traffic if time permits and an alternate course(s) of action. Any alternate course(s) of action the controller may recommend to the pilot will be predicated only on other traffic being worked by the controller.
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Oral Exam Questions a DPE Might Ask
Q1What is a Safety Alert and when will ATC issue one?
Per AIM 4-1-16, a Safety Alert is issued to pilots under ATC control when the controller judges the aircraft is at an altitude that places it in unsafe proximity to terrain, obstructions, or other aircraft. It's intended for time-critical situations; once issued, it's the pilot's prerogative to determine what action to take.
Q2What is MSAW and who receives this monitoring?
Per AIM 4-1-16, Minimum Safe Altitude Warning (MSAW) is an automated controller aid that alerts controllers when a tracked Mode C-equipped aircraft is below, or predicted to be below, a predetermined minimum safe altitude. It's provided to all IFR aircraft with operating Mode C, and to VFR aircraft with Mode C that request MSAW monitoring.
Q3What's the difference between a Terrain/Obstruction Alert and an Aircraft Conflict Alert?
Per AIM 4-1-16, a Terrain/Obstruction Alert warns of unsafe proximity to terrain or obstructions, detected primarily via Mode C altitude reports. An Aircraft Conflict Alert warns of unsafe proximity to another aircraft not under the controller's control; with the alert, the controller will, when feasible, provide the traffic's position and suggest an alternate heading or altitude.
Related Paragraphs in Chapter 4
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