Visibility is Controlling on Straight-in to Land

What is a Straight in Landing?

“Straight-in” when used in terms of an Intermediate Segment means that a Procedure Turn in not authorized or expected by ATC. Interesting to note is that the PT completion altitude must be within 1500 feet of the Minimum Descent Altitude. Straight-In to Land is what we are talking about here.

Inside both TERPS and PANS-Ops a Straight-in to land means that the angle of convergence of the Final Approach Course, FAC and the extended runway centerline must not exceed 30 degrees. The FAC should be aligned to intersect the extended runway centerline 3000 feet outward from the Landing Threshold Point,LTP. The minimum Required Obstacle Clerance,ROC in the primary area is 300 feet. The secondary area is 300 feet at the primary boundary, tapering uniformly to zero feet at the outer edge. 

First A Little History

Operational use of RVR reports began in 1955, but they were not available at most major airports until the early 1960s. Since 1989, all approach and landing operations using minimums below ½-sm visibility have been based on RVR reports.

In 1963, operating minimums were reduced to DH 200/RVR 1800 for two- and three‑engine airplanes (usually Category B or C) and DH 200/RVR 2000 for four‑engine airplanes (usually Category D). These reductions were based on the added requirement for enhanced in-runway lighting systems such as high-intensity touchdown zone (TDZ) and RCL lighting. 

In 1964, the minimums for runways not equipped with TDZ and RCL lights were established at DH 200/RVR 2400. 

By 1966, U.S. TERPS criteria was published and signaled a major change in the method for specifying operating minimums for approaches with vertical guidance. This evolved with the introduction of a DH with previous RVR concepts. This change eliminated the ceiling requirement by introducing a DH. This was necessary because of the limitations in the methods used to observe or measure ceiling and visibility. Often ceiling and visibility observations were taken several miles from the approach end of a runway, and as a result were frequently not representative of the actual conditions encountered during the final stages of an approach and landing, especially in rapidly changing or marginal weather conditions. 

It is easy to see (Catch the pun?) visual aids are critical in reducing landing minimums. These aids provide pilots with the necessary external visual references for manually controlling and maneuvering the aircraft during the final approach, flare, landing, and taxiing. 

Cats, no Dogs

Various categories of instrument approach operations have been established to accommodate a wide variety of airborne and ground- or space-based capabilities. These operational categories are necessary for granting credit to operators choosing to install airborne equipment with additional capabilities. These operational categories also provide the distinction between operational capabilities and ground support system configurations. CAT I, CAT II, and CAT III are the three basic categories of instrument approach operations.

CAT I Operations

Since 1988, a common CAT I operating minimum for all airplanes was established at DH 200/RVR 1800. CAT I operating minimums consist of a specified DA/DH that is not lower than the equivalent of 200 feet (60 meters) above the TDZ, and a visibility. This visibility can be a Runway Visibility Value, RVV not lower than ½ sm or an RVR that is not lower than 1800ft. 

In 2006, the lowest CAT I operating visibility minimums were revised to harmonize these minimums with the European Aviation Safety Agency (EASA). The majority of harmonized visibility minimums were based on a geometric calculation using the glidepath angle or published vertical angle, height above threshold (HATh), and length of instrument approach lighting. 

Standard lengths for four categories of instrument approach lighting were based on the minimum lengths of lighting systems in each category. RVRs for 200 feet HATh were calculated using a glidepath angle of 3 degrees. RVR values were restricted to a minimum of 1800 to retain operationally proven minimum RVRs. 

Use of visibility minimums below RVR 2400 requires operative TDZ and RCL lighting or use of an approved Head-Up Display, HUD , Flight Director, FD (except single-pilot), or autopilot coupled approach to DH.

SA CAT I

In 2009, FAA provided criteria for Special Authorization CAT I approaches with a DH as low as 150 feet HATh using radio altimeter minimums and a visibility minimum as low as RVR 1400 at runways with reduced lighting, provided an approved CAT II or CAT III HUD is used to DH. SA CAT I authorizes approaches to an RA DH as low as 150 feet and a visibility minimum as low as RVR 1400 to runways that do not have TDZ or RCL lighting when the approach is flown using an aircraft with a HUD to DH. There are no other special approvals or specific qualifications required for this approach.

Standard CAT II

CAT II operations are approach and landing operations conducted with a DH of less than 200 feet but not less than 100 feet and an RVR of not less than 1200ft.

CAT II RVR 1000

This CAT II approach means a DH as low as 100 feet and visibility minimums of RVR 1000 to runways that meet all CAT II equipment, performance, and lighting requirements. The operator must use either autoland or HUD to touchdown.

SA CAT II

CAT II approaches with a DH as low as 100 feet and visibility minimums of RVR 1200 at runways that do not meet all of the lighting requirements (Approach Lighting System With Sequenced Flashing Lights (ALSF)-2, TDZ, RCL lights) for standard CAT II. The operator must use either autoland or HUD to touchdown.

CAT III Operations

CAT III operations are separated into three subcategories: CAT IIIa, CAT IIIb, and CAT IIIc. These are approaches and landing operations with an RVR of less than 700 feet without a DH, with a DH of less than 100 feet or an Alert Height, AH that is typically between 50 and 200 feet. CAT Iliac is an approach and landing operation without a DH and without RVR limitations (zero-zero). CAT IIIc operations are currently not authorized.

What’s the Big Deal About a DA/DH?

The DA/DH concept evolved after the introduction of turbojets in 1958. It was established to resolve problems created by the use of a ceiling as an element of operating minimums, especially during rapidly changing weather conditions. A DA/DH is established to require that the pilot, at the specified height, decide whether adequate visual references are available or make a missed approach. 

The minimum heights or altitudes for IAP with vertical guidance (READ: Precision Approach Procedures/3D and Approach Procedures with Vertical Guidance, APV) 

can be specified as:  

  • Descent Altitude, DA
  • Descent Height, DH
  • Obstacle Clearance Altitude, OCA
  • Obstacle Clearance Height, OCH
  • Obstacle Clearance Limit, OCL
  •  

In the United States and certain foreign countries that use U.S. TERPS criteria, DA/DH is used. DA/DH is specified as a DA referenced to MSL for aircraft equipped with only barometric altimeters and as HAT. DA, DH, OCH, and OCL are used in most foreign countries and are established in accordance with various versions of ICAO PANS-OPS. DA and OCA are referenced to a barometric altitude (MSL). DH (in most countries), OCH, and OCL are referenced to a radio or radar height above either the elevation of the airport, the elevation of the TDZ, or the elevation of the landing threshold.

The decision that the pilot must make before passing DA/DH is not a commitment to land. It is a decision to continue the approach based on visual cues. This distinction is important since the possibility exists that, after passing DA/DH, visual cues may become inadequate to safely complete the landing, or the aircraft may deviate from the flightpath to a point where a safe landing cannot be assured. 

What’s the Big Deal About a MDA/MAP?

The minimum heights or altitudes for instrument approaches that do not have vertical guidance (READ “Non-Precision/2D”)                               can be specified as an:

  • Minimum Descent Altitude, MDA
  • Height Above Touchdown, HAT
  • Height Above Airport, HAA
  • Minimum Descent Height, MDH
  • Obstacle Clearance Altitude, OCA
  • Obstacle Clearance Height, OCH
  • Obstacle Clearance Limit, OCL
  •  

MDA and OCA are barometric flight altitudes referenced to mean sea level (MSL). HAT, HATh, HAA, MDH, OCH, and OCL are radio or radar altitudes referenced to either the elevation of the airport, the elevation of the TDZ, or the elevation of the landing threshold.

MDA, HAT, HATh, and HAA are used by the United States and certain foreign countries that use TERPS criteria. OCA, OCH, and OCL are used in most foreign countries and are established in accordance with ICAO Doc#8168 PANS-OPS. Although the current edition of ICAO PANS-OPS eliminated use of OCL, some countries still use OCL criteria from previous editions of PANS-OPS. Some countries, in addition to OCA and OCH, provide MDA and MDH. 

If the Minimum wasn’t Good Enough, It Would not be the Minimum

The basic objective of the controlling minimum concept is to provide reasonable assurance that once the aircraft begins the Final Approach Segment, the pilot will be able to safely complete the landing. RVR reports, when available for a particular runway, are the reports (controlling reports) that must be used for controlling whether an approach to, and landing on, that runway is authorized or prohibited.

This concept prohibits a commercial operation from continuing past the FAF or beginning the Final Approach Segment unless the reported visibility (RVV or RVR, if applicable) is equal to or greater than the authorized visibility (RVV or RVR) minimum for that IAP. The visibility/RVV/RVR needs to be reported to be at or above the controlling minimum when the pilot began the FAS, even though a later visibility/RVV/RVR report indicates a below-minimum condition. 

Part 135 operations must also comply with Part 91whether they are conducted in foreign countries or the United States (14CFR135.3(b)). Operations conducted under Part 135 need to account for 135.225. This will prohibit Part 135 operators from conducting “Look-See” approaches at any airport. This is significantly different from continuing a CAT I approach to DA/DH or MDA if the visibility/RVV/RVR was reported to be at or above the controlling minimum when the pilot began the Final Approach Segment. After beginning the Final Approach Segment, if a later visibility/RVV/RVR report indicates a below-minimum condition, Part 135 pilots are expected to continue the approach until reaching DA/DH. If the requirements of 14CFR91.175 are met, then continue visually and land even though the visibility/RVR is reported to be below the controlling minimum. This is similar to the ICAO Approach Ban procedure.

Interesting to note that Part 121 operations permit a pilot to begin the FAS, even though the reported visibility/RVV/RVR is below the controlling minimum, if the approach procedure is an ILS and the flight is actively monitored by a PAR. This means that Part 121 operators may conduct look-see approaches unless the rules of a foreign country (such as the United Kingdom) prohibit look-see approaches. If the rules of the foreign country prohibit look-see approaches, the controlling minimum concept applies in that country.

The controlling minimums concept is not applicable to Part 91 operators when determining if the pilot can continue past the FAF or begin the FAS. Parts 91 and 91K operations can begin an approach and continue to the DA/DH or the MDA and the MAP, even when the weather conditions are reported to be below the authorized IFR landing minimums. Upon arrival at the MDA and before passing the MAP, or upon arrival at the DA/DH, the approach may be continued below DA/DH or MDA to the runway if the seeing-conditions required by 14CFR91.175. For HUD/EFVS operations 14CFR176 is the operable regulation.

ICAO’s “Approach Ban” Procedure, Commercial Ops Applied to Private Ops

Annex 6, Part 1 “Commercial” and Part 2 “International General Aviation” (READ: Part 91 Ops) are almost identical in this. A flight shall not be continued towards the aerodrome of intended landing, unless the latest available information indicates that at the expected time of arrival, a landing can be effected at that aerodrome or at least one destination alternate aerodrome, in compliance with the operating minima.

An instrument approach shall not be continued beyond the outer marker fix in case of precision approach, or below 300 m (1000ft) above the aerodrome in case of non-precision approach, unless the reported visibility or controlling RVR is above the specified minimum.

If, after passing the outer marker fix in case of precision approach, or after descending below 300 m (1000ft) above the aerodrome in case of non-precision approach, the reported visibility or controlling RVR falls below the specified minimum, the approach may be continued to DA/H or MDA/H. In any case, an aeroplane shall not continue its approach-to-land at any aerodrome beyond a point at which the limits of the operating minima specified.

Controlling RVR means the reported values of one or more RVR reporting locations (touchdown, mid-point and stop-end) used to determine whether operating minima are or are not met. Where RVR is used, the controlling RVR is the touchdown RVR, unless otherwise specified by State criteria.

So USA Commercial and Private oeprators in a country that uses ICAO Annex 6 without exception to Paragraph 4.4.1 in Part 1 or Paragraph 2.2.2.2 in Part 2 are not allowed to begin an approach if the controlling RVR is below the minimum for the approach. Even if you start the Final Approach Segment and RVR is reported below the minimum you are not permitted to descend to the published minimum. Instead both Commercial and Private operators are expected to descend not lower than 1000ft above the field, follow the lateral guidance to the Missed Approach Point then, comply with eth missed approach procedures.

Keep in mind that almost all the simulator training and cockpit procedures in the US are built around the idea of starting the go-around from the DA/DH and accelerating to 250KIA and turning downwind for another approach. This is in direct conflict with the Approach Ban and can lead USA pilot into busting speed and altitude constraints in a foreign country’s Missed Approach Procedure.

Bottomline
  • RVR, then RVV, then Tower Visibility are the controlling minimums on Straight-in to Landing Approaches
  • Ceiling is not Controlling on Straight-in to Landing Approaches
  • ICAO’s Approach Ban is similar to Commercial Ops in the US
  • Missed Approach training in the US set’s up operators for a bust outside the US
  • Large parts of this paper came from quotes found inside OPSSpecc052, FAA Order 8900.1, Volume 4, Chapter 2, Section 1, Paragraphs 4-150 thru 4-162 an ICAO Annex 6, Parts 1 and 2.