Commercial and Business Aviation Advisory Circulars

Stabilized Constant Descent Angle Non-Precision Approach

INTRODUCTION

Significant changes to the Approach Ban will come-into-force December 1, 2006, which will affect commercial operators. The aim of this Commercial and Business Aviation Advosiry Circular (CBAAC) is to describe Stabilized Constant Descent Angle (SCDA) Non-Precision Approach (NPA) procedures. Operators, who are authorized through Operations Specification (Ops Spec) and whose operation meets the specified conditions, may conduct an approach in lower visibility conditions by using SCDA NPA procedures. The material in this CBAAC is referenced in CBAAC 0237, Changes to the Approach Ban.

APPLICABILITY

This CBAAC is primarily applicable to operators under Subparts 703, 704 and 705 of the Canadian Aviation Regulations (CARs) who may be authorized through Ops Spec 019, 303, or 503 respectively to conduct SCDA NPA procedures at reduced approach ban visibility values RVR, or ground visibility at aerodromes south of 60 degrees North Latitude (60oN Lat), and to use the Minimum Descent Altitude (MDA) as a Decision Altitude (Height) DA(H).

REFERENCES

• Section 101.01 of the CARs
• Subparts 703/723(A), 704/724(A) and 705/725 of the CARs
• Canada Gazette I, Vol. 138, No. 7
• CBAAC 0237, Changes to the Approach Ban
• CBAAC 0239, Pilot Monitored Approach (PMA)

TERMINOLOGY & DEFINITIONS

• APV - Approach Procedure with Vertical guidance
• baro-VNAV - Barometric Vertical Navigation
• CFIT - Controlled Flight Into Terrain
• DA(H) - Decision Altitude (Height)
• FAF - Final Approach Fix
• FPA - Flight Path Angle
• GPS - Global Positioning System
• IAP - Instrument Approach Procedure
• ICAO - International Civil Aviation Organization
• IFR - Instrument Flight Rules
• ILS - Instrument Landing System
• LNAV - Lateral Navigation
• LPV - Localizer performance with vertical navigation
• MSA - Minimum Sector Altitude
• MDA - Minimum Descent Altitude
• MAP - Missed Approach Point
• nm - Nautical mile
• NPA - Non-Precision Approach
• Ops Spec - Operations Specification
• PMA - Pilot Monitored Approach
• PT - Procedure Turn
• RNAV - Area Navigation
• ROC - Required Obstacle Clearance
• SCDA - Stabilized Constant Descent Angle
• SOPs - Standard Operating Procedures
• VNAV - Vertical navigation
• VS - Vertical Speed
• VSI - Vertical Speed Indicator
• “SCDA Non-Precision Approach” (SCDA NPA) means stabilized constant-descent-angle non-precision approach.
• “Stabilized approach” means a final approach flown to achieve a constant rate of descent, at an approximate 3 degree descent flight path angle, with stable airspeed, power setting, and attitude, with the aircraft configured for landing.

BACKGROUND

In response to the significant number of transport category CFIT accidents, an international effort has been made to prevent these accidents. When compared with ILS, NPAs greatly increase the risk of approach and landing CFIT accidents. A Flight Safety Foundation study of accidents and available approach and landing aids determined that the accident risk while flying an NPA was 5 times greater than that associated with flying a precision approach. An NPA does not provide vertical guidance that ends at the runway. As a result, the flight crew must more actively navigate the aircraft vertically during the approach. The chance of error by the crew is greater because of increased workload and the additional need for situational awareness. The increased workload and need for situational awareness is related to when to commence a descent from, and when to level off at the published minimum IFR altitudes.

An exemption to paragraph 602.128(2)(b) of the CARs has been in place since November 1996 to permit operators to use SCDA NPA procedures allowing MDA to be used as a DA(H). The exemption is required in order to accommodate the altitude loss below MDA that will likely occur during a missed approach following a SCDA NPA procedure. With the coming-into-force of the amendments to the Approach Ban, SCDA NPA procedures will be incorporated into the CARs making an exemption unnecessary for Canadian commercial operators.

NPA – STEP -DOWN TECHNIQUE

NPAs have traditionally been flown using step-down techniques that result in an unstabilized approach. The descents and level offs result in significant changes in power settings and pitch attitudes, and in some aircraft may prevent the landing configuration from being used until landing is assured. The aircraft is flown to descend to and then level off at the minimum IFR attitudes published for the IAP. In the final segment of the IAP, the aircraft is flown to cross the FAF at the minimum crossing altitude. After crossing the FAF, the aircraft is descended at a rate-of-descent such that the aircraft can be leveled at MDA prior to the MAP. In minimum weather conditions, in order to have an effective chance of completing a normal descent and landing after reaching MDA, the aircraft should be level at the MDA at a distance equal to or greater than the published visibility minima prior to the MAP. In using the step-down technique, the aircraft flies an unstable vertical profile during the final approach segment as it descends and levels off at the minimum altitudes published for the approach, and then if visual descends from MDA to landing.

Using the step-down technique, the aircraft is flown level at minimum altitudes for extended periods of time. The aircraft descends to the initial approach minimum altitude from the enroute structure. The route flown may be a transition route, a direct routing descending to and leveling at the MSA, or using the PT for a course reversal descending to and leveling at the PT altitude. Both MSA and minimum PT altitude provide 1,000 feet ROC. Once inbound to the FAF on the intermediate segment, the aircraft is descended to and leveled at the minimum FAF crossing altitude (provides 500 feet of ROC). After crossing the FAF and on the final approach segment, the aircraft is descended to and leveled at the MDA (provides at least 250 feet of ROC.) The aircraft is flown level at MDA until the runway environment is sighted and a descent to landing can be made, or it reaches the MAP where a missed approach is commenced.

In using the step-down technique, the aircraft flies a series of unstable vertical profiles during the final approach segment as it descends and levels off at the minimum IFR altitudes published for the approach, and then if visual descends from MDA to landing. Using the step-down technique, the aircraft is flown for extended periods at the minimum altitudes, exposing the aircraft to extended periods of time at minimum altitudes above terrain and obstacles. A premature descent or a missed level off exposes the aircraft to a CFIT accident potential.

NPA – STABILIZED FINAL APPROACH

The need for a stabilized final approach during NPAs has been recognized by the International Civil Aviation Organization (ICAO) CFIT Task Force as a means to prevent CFIT accidents. The step-down technique (presumed by the procedure design) may have been appropriate to early piston transport aircraft, but most modern jet transport aircraft are much faster, heavier, have greater inertia and are less maneuverable than early aircraft. These factors make late changes in vertical profile undesirable and even dangerous. Many operators require their crews to use a stabilized technique, which is entirely different from that envisaged in the original NPA procedure design. A stabilized approach is flown to achieve a constant rate of descent, at an approximate 3-degree descent flight path angle, with stable airspeed, power setting, and attitude, with the aircraft configured for landing. The safety benefits derived from a stabilized final approach during an NPA have been recognized by most organizations including ICAO, the Federal Aviation Administration, and Transport Canada Civil Aviation.

SCDA NPA

The aim of an SCDA NPA procedure is to minimize the vertical manoeuvring required while flying most NPAs from the final approach segment through to touch down. The goal is to achieve a final approach vertical path that approximates that of a normal glide path. An SCDA NPA procedure allows certain NPAs to be flown using the MDA as a DA(H).

Using the SCDA NPA procedure, the aircraft is not flown at minimum altitudes for extended periods of time. If the route flown intercepts the intermediate approach segment, then a higher enroute altitude can be maintained until an approximate 3-degree vertical descent path is intercepted and a continuous descent to MDA can be made. If a course reversal using a PT is required, the aircraft can be flown at or above the minimum PT altitude until the 3-degree vertical descent path is intercepted and a continuous descent to MDA can be made. No later than crossing the FAF, the aircraft descends stabilized on the planned constant descent angle configured for landing, with stable airspeed, power setting, and attitude. The aircraft is descended towards MDA until the runway environment is sighted and the descent continued to landing, or until it reaches MDA (treated like a DA(H)) where a missed approach is commenced.

Figure 1 (Annex A) compares the vertical descent profiles of a NPA flown using a SCDA NPA technique to the traditional step-down technique. A typical NPA provides 1,000 feet of ROC in the initial segment and the procedure turn, 500 feet of ROC in the intermediate segment, and at least 250 feet of ROC in the final segment.

The SCDA NPA procedure is normally applicable to NPAs that meet the following three criteria:

• The approach is flown to straight-in minima. (SCDA is not normally applicable to circling approaches where the aircraft has to level at MDA and a circling procedure flown before a descent to landing can be made.)
• The approach design should permit a final approach segment descent angle of 2.9 to 3.5 degrees. (NPA approach design criteria permit a final approach segment with a descent gradient up to 3.77 degrees. Therefore, there are a few NPAs with steep descent gradients that exceed 3.5 degrees, for which the SCDA technique would not be appropriate. New approach design criteria for NPAs with vertical guidance limit the descent angle to 3.5 degrees or less.)
• The final approach course shall not be more than 15 degrees from runway centreline. (NPA design criteria permit straight-in minima to be published for IAPs with final approach courses up to 30 degrees from the runway centreline. The SCDA approach procedure is not normally applicable to straight-in approaches with final approach courses that exceed 15 degrees from the runway centreline, in order to reduce the requirement for lateral manoeuvring during the transition from approach to landing.)

An SCDA NPA final approach descent is flown with a planned SCDA of not less than 2.9 degrees and not greater than 3.5 degrees from the FAF to a nominal landing runway threshold crossing height of 50 feet. The angle flown is selected to ensure that minimum FAF crossing altitude and any step-down altitudes between the FAF and the MAP are respected.

The final descent path can be flown using baro-VNAV guidance, FPA guidance, VS based on groundspeed, and/or check altitudes based on distance from touch-down.

SCDA TRAINING PROGRAM

The operator should ensure that flight crews receive ground and simulator or flight training that addresses SCDA NPA procedure proficiency within their initial and recurrent training programs to include the following subjects.

The operation and use of aircraft altitude pre-selector and computer-generated approach slope systems (such as baro-VNAV or FPA) or other methods of computing SCDA NPA path to the 50 feet nominal threshold crossing height, should be thoroughly understood and trained.

The effects of horizontal position error and temperature on the vertical path, whether it is derived from baro-VNAV, inertial, or altimeter/VSI reference, should be addressed.

Temperature corrections to MDA and other published/procedural altitudes should be made to ensure that the true vertical flight path remains between 2.9 and 3.5 degrees.

If waypoint information (position and altitude) from a navigation database is used, it should be verified against an independent source.

Any altitudes at step-down fixes between the FAF and the MAP must be respected. The planned final descent angle should take minimum altitudes at step-down fixes into account so that the approach remains stabilized in the final segment.

When flown correctly, the position where a missed approach is commenced following an SCDA NPA to MDA will occur before the published MAP. In Figure 1 (Annex A), the MDA should be reached approximately 1 nm before the published MAP. Therefore, the missed approach climb will normally occur some distance before reaching the published MAP.

It is important to note that the missed approach climb can safely begin at any point prior to the MAP; however, the requirement to commence the horizontal (lateral) navigation portion of the published missed approach procedure begins at the MAP in all cases. It may be essential for obstacle clearance to delay any turns stated in the published missed approach procedure until the aircraft crosses the MAP.

When a missed approach is commenced at MDA, the aircraft will dip below MDA as it transitions to a climb. Several factors affect the amount of altitude lost during the initiation of a missed approach from a descent. These factors include:

• Time required for a decision (reaction time);
• Rate of descent at commencement of the missed approach;
• Pilot technique;
• Aircraft performance; and
• Baro altimeter lag.

SOPs

The operator should have SOPs incorporating SCDA NPA procedures. These procedures should cover crew coordination during all phases of the approach, including reaching MDA and during the execution of a missed approach.

The SOPs should include a specified amount to be added to the MDA to compensate for the additional height loss during the go-around initiation during approaches where:

• There is a failure of an aircraft system;
• The aircraft is above normal maximum landing weight;
• The aircraft landing weight is limited by aborted landing climb performance; or
• Height loss could be expected to be larger than normal.

SUMMARY

The aim of an SCDA NPA technique is to minimize the vertical maneuvering required while flying most NPAs. The goal is to achieve a final approach vertical path that approximates that of a normal glide path. The SCDA NPA procedure can be applied to the majority of NPAs. An SCDA NPA final approach descent is flown with a planned descent angle of approximately 3 degrees from the FAF to a runway threshold crossing height of approximately 50 feet. The SCDA NPA procedure reduces pilot workload by reducing the number of positions required to commence a descent from, and when to level off at the published minimum IFR altitudes. The SCDA NPA technique’s vertical flight path increases the aircraft’s altitude above terrain and obstacles for most of the approach, and reduces the period of time the aircraft is flown at minimum altitudes. The need for a stabilized final approach provided by the SCDA NPA procedure during NPAs has been recognized by the ICAO CFIT Task Force as a means to prevent CFIT accidents.

In addition to the safety benefits, air operators, who are authorized through Ops Spec and whose operation meets the specified conditions, may conduct an approach in lower visibility conditions by using SCDA NPA procedures.  Refer to the material in CBAAC 0237, Changes to the Approach Ban for further information.

Original signed by

D.B. Sherritt
Director
Commercial & Business Aviation

Annex A

Figure 1 – Vertical Descent Profile Comparison – SCDA to Step-Down