What is CO55 - Derived Alternate Minimums (1Nav2Nav) 

To understand what a derived alternate minimum is, we first have to deep dive into §91.169.

Part 91.169 explains the order for minimum weather values needed for an alternate airport:

Unless otherwise authorized by the Administrator, no person may include an alternate airport in an IFR flight plan unless appropriate weather reports or weather forecasts, or a combination of them, indicate that, at the estimated time of arrival at the alternate airport, the ceiling and visibility at that airport will be at or above the following weather minima:

1. IAP charts will have a published non-standard alternate minimums. Without CO55, this will be the weather minimums used.
   • • Found in IAP charts with a "A" triangle symbol in the briefing section, then found in the Terminal Procedures Publication.

   

   

2. If the destination airport does NOT have non-standard alternate minimums, we will use the standard alternate minimums.
   • For precision approach - 600 feet ceiling, 2 sm visibility
   • For non precision approach - 800 feet ceiling, 2 sm visibility

To understand why we need an alternate airport, read more about destination alternates or takeoff alternates.

Derived Alternate Minimums

The first sentence of Part 91.169, "Unless otherwise authorized by the Administrator", is the why we have CO55. CO55 is an exception to give Part 121 operators the authority to have derived alternate minimums. 

Why?

The main reason to have derived alternate minimum is for Part 121 the ability to have lower alternate weather minimums in respect to the navigational facilities for the alternate airports.

The rule, often called "1Nav2Nav" states that:

• If an alternate airport has only one navigational facility (ILS, RNAV, VOR, LOC, etc) for a straight in approach
  • Add 400 feet to the MDA or DH
  • Add 1sm or 1600m RVR to the visibility
• If an alternate airport has two or more navigational facility for a straight in approach on different runways
  • Add 200 feet to the MDA or DH
  • Add 1/2sm or 800m RVR to the visibility

Example

Lets look at an example, assume we are a category D with auto landing and KPHL is our alternate airport. KPHL is landing air planes at Runway 9L and we are planning to use the S-ILS 9L* approach. 

Earlier in the post, since the ILS Runway 9L for KPHL has a nonstandard alternate minimum. We CANNOT use the standard alternate minimum for a precision approach of 600 ft and 2 sm.

Using the Terminal Procedure Publication, we found that the non-standard alternate minimum for ILS Runway 9L is 800 ft ceiling, and 2 and 1/4 mile visibility. 

If we have the ability to exercise CO55 and have the authority to use the derived alternate minimums, we are able to invoke the "1Nav2Nav" rule to lower our alternate minimums. 

Since we know KPHL has more than one navigational facility, and we are using the S-ILS 9L* approach, we can add 200ft and 1/2 statue mile to the DH and visibility.

Therefore our new alternate minimum with CO55 is now 400 ft, 1sm, much lower than the nonstandard alternate minimum given to us by the Terminal Procedure Publication.