Minimum Sector Altitude (MSA), Explained For Instrument Pilots

The Short Definition

The Pilot/Controller Glossary defines the charted MSA as altitudes "depicted on approach charts or on a DP graphic chart which provide at least 1,000 feet of obstacle clearance within a 25-mile radius of the navigation facility, waypoint, or airport reference point upon which the MSA is predicated." The next sentence is the one that matters in the airplane: "MSAs are for emergency use only and do not necessarily assure acceptable navigational signal coverage."

The ICAO wording says the same thing in metric first. It is the lowest altitude that may be used under emergency conditions, with a minimum clearance of 300 m (1,000 feet) above all obstacles. The area is a sector of a circle of 46 km (25 NM) radius, centered on a radio aid to navigation.

AIM 5-4-5 adds two details the glossary leaves out. The radius is normally 25 NM, but on conventional procedures it may be expanded to 30 NM if that is needed to take in the airport's landing surfaces. And the same circle can appear on a departure procedure chart as well as an approach chart.

The glossary entry has a first item that has nothing to do with the chart: the minimum safe altitudes of 14 CFR part 91, the rule that says how low you may fly over people and property. Same words, different thing. The section on other minimum altitudes below puts the two side by side.

Where MSA Appears On The Chart

The MSA sits in the plan view, the overhead drawing of the approach. AIM 5-4-5 says the depiction carries four things: the identifier of the center point, the radius, a depiction of the sector or sectors, and the minimum altitude for each in feet above mean sea level. Read them in that order.

MSA Vs The Other Minimum Altitudes

Instrument charts and rules carry several minimum altitudes, and they are easy to mix up. Here they are together, each from the FAA source that defines it.

The pair that causes the trouble is the first row and the last. 91.119 protects people and property under you in every phase of flight except takeoff and landing. The MSA protects you from obstacles within 25 NM of a point, in an emergency, on instruments. Over a congested area, 91.119 asks for 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet; over open water or sparsely populated areas it asks you to stay 500 feet from any person, vessel, vehicle or structure. Our guide to flying in Los Angeles airspace applies 91.119 to the city in its section on how low you may fly.

All of these are floors. How high airplanes actually fly above them, from a trainer over the Valley to an airliner at cruise, is the subject of How High Do Planes Fly.

The TAA deserves a second look on RNAV charts. Where a procedure publishes one, AIM 5-4-5 says its altitudes replace the MSA, and the IPH tells you to use the TAA altitudes in place of the MSA in an emergency. For the minimums at the bottom of an approach, the DA and the MDA, see our page on FAR 91.175. Short definitions of every term in this table are in the aviation glossary.

Why Terrain Makes This Real Around The San Fernando Valley

Van Nuys Airport sits on the floor of the San Fernando Valley, a Class D airport under the Los Angeles Class B shelf, with parallel runways of 8,001 and 4,000 feet. The Valley is ringed by high ground: the Santa Monica Mountains to the south, the Santa Susana Mountains to the northwest, where our practice area lies, and the San Gabriel Mountains to the northeast, beyond Whiteman Airport six miles away.

High ground on three sides is the reason to read the MSA circle on any chart around here before you need it. We are not going to print MSA values from memory on a web page, and you should not use them from one. The only MSA to trust is the one on the current chart for the procedure you are flying, from the FAA's Terminal Procedures Publication, checked on the ground before the flight.

The same ridges shape every flight out of Van Nuys, visual or instrument. If you have only seen them from the freeway, a $229 discovery flight puts you in the left seat above the Valley for an hour. A two-hour first lesson for $349 is the first entry in a logbook that can end with an instrument rating.

How MSA Shows Up In Training And On The Written

The ACS that governs the instrument checkride, FAA-S-ACS-8C, never uses the words minimum safe altitude. The nearest element is IR.I.C.K2 under cross-country flight planning: altitude selection accounting for terrain and obstacles, glide distance of the airplane and IFR cruising altitudes. Appendix 1 of the ACS says knowledge and risk management elements are primarily evaluated during the knowledge testing phase, so that element belongs on your written study list. Controlled flight into terrain appears as a risk element in the circling approach task, IR.VI.D.R6.

The chart side of it comes from AIM 5-4-5 and chapter 4 of the IPH, both summarized above. In the airplane or the simulator, you meet the MSA every time you brief the plan view of an approach. The other plan-view symbol worth knowing cold is the hold, and our guide to holding patterns covers it. The knowledge test itself is covered on the instrument written and the ACS.

If you are training for the rating, our instrument package is $11,700: 20 hours in the airplane, 20 in the Redbird MCX full-motion simulator, 40 hours of dual instruction and 10 of ground. The course is explained on instrument rating in Los Angeles. If you would rather fly an approach before you commit to anything, an hour in the simulator with an instructor is $189.

Questions About Minimum Sector Altitude

Is Minimum Sector Altitude The Same As The Minimum Safe Altitude In 91.119?

No. The Pilot/Controller Glossary lists them as separate items under one heading. 91.119 governs all flight except takeoff and landing, and its paragraph (a) also requires, anywhere, an altitude that allows an emergency landing without undue hazard to people or property if a power unit fails.

How Far Does An MSA Protect You?

Normally 25 NM from its center point, with at least 1,000 feet of obstacle clearance. The ICAO definition gives the same radius as 46 km and the clearance as 300 m.

Can I Descend To The MSA On A Normal Approach?

No. The MSA is for emergency use. On a normal approach, 14 CFR 91.177 holds you to the minimum altitudes prescribed in parts 95 and 97 of the regulations, and part 97 is where standard instrument approach procedures are published.

Does The MSA Assure Radio Or GPS Reception?

No. The glossary says MSAs do not necessarily assure acceptable navigational signal coverage. The altitude that does promise acceptable signal coverage en route is the MEA.

What Is An Emergency Safe Altitude?

An altitude giving 1,000 feet of obstacle clearance, or 2,000 feet in designated mountainous areas, within 100 NM of the facility or waypoint at its center. The glossary says ESAs are normally used only in military procedures, and they are labelled Emergency Safe Altitudes on the chart.

What Replaces The MSA On Many RNAV Approaches?

The Terminal Arrival Area. AIM 5-4-5 describes it as a transition from the en route structure to the terminal environment, divided into straight-in, left-base and right-base areas, and where one is published its altitudes replace the MSA.

Keep Reading

• Instrument Flying Skills: The Scan, Approach Charts And Holds
• Holding Patterns: Entries, Timing, Speeds And Wind Correction
• FAR 91.175: Takeoff And Landing Under IFR
• Flying In Los Angeles Airspace: Class B, The Corridors And Van Nuys
• Aviation Glossary: 150 Terms Every Student Pilot Meets
• Instrument Rating In Los Angeles