WEATHER AND VISIBILITY MINIMUMS

TAXIING, TAKE OFF, APPROACHES

In this newsletter we will discuss the minimums (mins) for operations in aviation. We have looked at some of the weather makers in previous newsletters: Fronts, Pressure and Wind Systems, and Restrictions to Visibilities. Now let’s connect these restrictions to flight operations and the legalities involved.

Operating rules are set by Title 14 Code of Federal Regulations (CFR), Federal Air Regulations (FAR’s) for all flight operations. We will focus on what we need to know for FAR part 91 Operations Specifications (OpSpecs), which we as Pilots in Command (PIC’s) are responsible to follow. I will compare what commercial flight operations use for OpSpecs with Part 91 and GA regulations. FAR Parts 121 and 135 flight ops have an outstanding safety record in comparison to GA’s flight safety record. FAA and Company regulations often further restrict some operations in low visibilities, and/or ceilings. (Instrument Procedures Handbook, FAA-H-8083-16B; OpSpecs, pg 1-8).

FAA minimums for taxi, takeoff and landing under limited visibilities, are set as standard operating rules, such as:

·      Taxiing on an airport: usually requires a ¼ mile visibility or RVR of 1200, for FAR Part 121, 125,and 131 ops, when reported visibility is below 1200 a Surface Movement Guidance and Movement System (SMGCS) is required (AC 120-57B). Part 91 with a Tower: a pilot must have clearance to taxi and cross any runways, Non-Tower: PIC’s discretion, however if the visibility is below 1200 RVR it’s probably not a good idea to taxi and takeoff. 

·      Takeoffs: minimums are often changed or raised by local conditions like terrain. Commercial Carriers ½ statute mile (sm) or less. Part 91: Single Eng A/C 1 sm, Twin Eng ½ sm (FAR 91.175). The PIC must take into consideration if an aircraft emergency occurs after takeoff will he/she have the required visibility and ceiling to return to the airport for a safe landing.

Types of approaches: Mostly used in the US (Precision and Non Precision)

• Non Precision: A standard instrument approach in which no electric glide slope is provided. Includes: VOR, ADF, Circling approaches (CTL). 

• Circle to Land (CTL): usually has higher minimums than the CAT I, this type of approach has proven to be DANGEROUS, the airlines have stopped circling approaches. For a GA pilot I must WARN you about this approach, my Will-ism is DON’T DO IT! Refer to: AIM (Chapter 5, Air Traffic Procedures [pg 5-4-21, f], Circling Minimums) states: “Circling may require maneuvers at low altitude, at low airspeed, and in marginal weather conditions”.

• Precision: Localizer with a Glide Slope or Precision Approach Radar (PAR)

  • Category I approaches: (CAT I)  the pilot must know the reported visibility to begin an approach, mins will depend on the airport and surrounding terrain. Mins normally require ½ mile visibility and 200 ft ceilings.

  • Category II: (CAT II): Approach Height (HAT) of not less that 100’ Decision Height (DH). Flown on autopilot and monitored by pilots. Not used in GA.

  • Category III, IIIa, IIIb: are used at certain airports, terrain around airport must be relatively flat, and Auto Land is performed with the Auto-Pilot.  No ceiling is required but visibilities can be as low as 300’ RVR. No DH Minimum. Not used in GA (Problem here is taxiing after landing!)

• Visibility Minima nomenclature: refer 14CFR FAR Part 97, AIM Chapter 5.

  • DA Decision Altitude: altitude determined by sensitive altimeter

    DH Decision Height: height above runway determined by a radio altimeter

  • MDA Minimum Descent Altitude: read on altimeter for Non Precision approach, Height above airport (usually center point of airport)

  • HAA Height above airport average height of airport above msl, used for circling approaches

  • HAT Height above touchdown, the highest elevation for the runway of intended landing, within the first 3,000 ft, used for all Straight in approaches. 

  • RVR Runway Visual Range: an instrument determined visibility in touchdown area of runway

Again, KNOW before you GO! The weather, what is happening at your airport of intended landing, (NOTAMS), where are your alternates, the topography, how much fuel will you need, NEVER land with less than 30 minutes of fuel. A well flown instrument approach is personally very rewarding, and fun.

Fly Safely                  Know the Weather                        Plan Ahead               FUEL

Until next time,

Captain Will Rondeau

References

FAA visibility minimums: https://fsims.faa.gov/wdocs/orders/8260_3b-chg20.htm

FAR AIM Federal Aviation Regulations Aeronautical Manual. FAR Parts: 91, 121, 135. AIM Chapters: 1, 2, 3,5. 

FAA Operations Manual: https://faa-h-8083-16b_chapter_4.pdf

AIM Chpt 4 Air Traffic Control, 4-3-19 Taxi During Low Visibility (pg. 695).