Minimum safe altitudes (MSAs) are published for emergency use on IAP charts. For conventional navigation systems, the MSA is normally based on the primary omnidirectional facility on which the IAP is predicated. The MSA depiction on the approach chart contains the facility identifier of the NAVAID used to determine the MSA. For RNAV approaches, the MSA is based on either the runway waypoint (RWY WP) or the missed approach waypoint (MAWP) for straight-in approaches, or the airport waypoint (APT WP) for circling only approaches.For RNAV (GPS) approaches with a terminal arrival area (TAA) the MSA is based on the IAF waypoint.

MSAs are expressed in feet above MSL and normally have a 25 NM radius. This radius may be expanded to 30 NM if necessary to encompass the airport landing surfaces. Ideally, a single sector altitude is established and depicted on the planview of approach charts. When necessary to maintain clearance from obstructions, the area may be further sectored and as many as four MSAs established. When established, sectors may be no less than 90°in spread. MSAs provide 1,000 feet clearance over all obstructions but do not necessarily assure acceptable navigation signal coverage.


Another important altitude that should be briefed during an IAP briefing is the FAF altitude, designated by the cross on a nonprecision approach, and the lightning bolt symbol designating the glide slope intercept altitude on a precision approach. Adherence to and crosscheck of this altitude can have a direct effect on the success of an approach.

Proper airspeed, altitude, and configuration, when crossing the FAF of a nonprecision approach, are extremely important no matter what type of aircraft is being flown. The stabilized approach concept, implemented by the FAA within the SOPs of each air carrier, suggests that crossing the FAF at the published altitude is often a critical component of a successful nonprecision approach, especially in a large turbojet aircraft.

The glide slope intercept altitude of a precision approach should also be included in the IAP briefing. Awareness of this altitude when intercepting the glide slope can ensure the flight crew that a “false glide slope” or other erroneous indication is not inadvertently followed. Many air carriers include a standard callout when the aircraft passes over the FAF of the nonprecision approach underlying the ILS. The pilot monitoring (PM) states the name of the fix and the charted glide slope altitude, thus allowing both pilots to crosscheck their respective altimeters and verify the correct indications.


MDA and DA are referenced to MSL and measured with a barometric altimeter. CAT II and III approach DHs are referenced to AGL and measured with a radio altimeter.

The height above touchdown (HAT) for a CAT I precision approach is normally 200 feet above touchdown zone elevation (TDZE). When a HAT of 250 feet or higher is published, it may be the result of the signal-in-space coverage, or there may be penetrations of either the final or missed approach obstacle clearance surfaces (OCSs). If there are OCS penetrations, the pilot will have no indication on the approach chart where the obstacles are located. It is important for pilots to brief the MDA, DA, or DH so that there is no ambiguity as to what minimums are being used. These altitudes can be restricted by many factors. Approach category, inoperative equipment in the aircraft or on the ground, crew qualifications, and company authorizations are all examples of issues that may limit or change the height of a published MDA, DA, or DH.

The primary authorization for the use of specific approach minimums by an individual air carrier can be found in Part C–Airplane Terminal Instrument Procedures, Airport Authorizations and Limitations, of its FAA approved OpsSpecs. This document lists the lowest authorized landing minimums that the carrier can use while conducting instrument approaches. Figure 5-14 shows an example of a carrier’s OpsSpecs that lists minimum authorized MDAs and visibilities for nonprecision approaches.

Figure 5-14. Authorized Landing Minimums for Nonprecision Approaches.

As can be seen from the previous example, the OpsSpecs of this company rarely restrict it from using the published MDA for a nonprecision approach. In other words, most, if not all, nonprecision approaches that pilots for this company fly have published MDAs that meet or exceed its lowest authorized minimums. Therefore the published minimums are the limiting factor in these cases.

For many air carriers, OpsSpecs may be the limiting factor for some types of approaches. NDB and circling approaches are two common examples where the OpsSpecs minimum listed altitudes may be more restrictive than the published minimums. Many Part 121 and 135 operators are restricted from conducting circling approaches below 1,000-feet MDA and 3 SM visibility by Part C of their OpsSpecs, and many have specific visibility criteria listed for NDB approaches that exceed visibilities published for the approach (commonly 2 SM). In these cases, flight crews must determine which is the more restrictive of the two and comply with those minimums.

In some cases, flight crew qualifications can be the limiting factor for the MDA, DA, or DH for an instrument approach. There are many CAT II and III approach procedures authorized at airports throughout the U.S., but Special Aircraft and Aircrew Authorization Requirements (SAAAR) restrict their use to pilots who have received specific training, and aircraft that are equipped and authorized to conduct those approaches. Other rules pertaining to flight crew qualifications can also determine the lowest usable MDA, DA, or DH for a specific approach. Parts 121.652, 125.379, and 135.225 require that some pilots-in-command, with limited experience in the aircraft they are operating, increase the approach minimums and visibility by 100 feet and one-half mile respectively. Rules for these “high-minimums” pilots are usually derived from a combination of federal regulations and the company’s OpsSpecs. There are many factors that can determine the actual minimums that can be used for a specific approach. All of them must be considered by pilots during the preflight and approach planning phases, discussed, and briefed appropriately.