Potential safety issues associated with RNAV approaches which have track changes at the final approach fix/final approach waypoint (FAF/FAWP) - Civil Aviation Safety Alerts (CASA) No. 2018-01

Attention:

All pilots, flight dispatchers, air operators, private operators and foreign operators conducting flight operations under instrument flight rules in Canadian airspace. 

File Classification No. :	Z-5000-35
RDIMS No. :	13857683
Document No. :	CASA 2018-01
Issue No. :	01
Effective Date:	2018-03-15

Purpose:

The purpose of this Civil Aviation Safety Alert (CASA) is to alert pilots, flight dispatchers, air operators, private operators and foreign air operators of potential safety issues associated with the varied capabilities of some older generation Autopilot/Flight Director (AP/FD) systems when conducting RNAV approaches which have been designed with a track change at the final approach fix (FAF) /final approach waypoint (FAWP).

This CASA provides essential information respecting these safety issues and outlines the steps necessary to address them.

Background:

NAV Canada has been introducing RNAV approaches with turns at the FAF*. Track changes at the FAF are utilized where terrain, obstacles, or restricted airspace prevent a straight line transition from the intermediate segment to the final approach segment.

* For the purpose of this CASA, the term FAF also means FAWP.

Track changes at the FAF for these RNAV approaches may be up to 15° for localizer performance with vertical guidance (LPV) and lateral navigation/vertical navigation (LNAV/VNAV) approaches, and up to 30° for LNAV and localizer performance without vertical guidance (LP) approaches. In some cases the approach geometry results in relatively short final approach segments.

The AP/FD systems in contemporary (modern) aircraft are normally able to execute the turn at the FAF correctly and smoothly within the required airspace. These aircraft will stay coupled to the Flight Management System (FMS) roll and pitch steering commands in the appropriate lateral and vertical AP/FD modes (LNAV and VNAV) and Approach (APR), and track correctly through the track change.

Aircraft equipped with some older generation FD/AFCS may not have the same capability as more modern versions. Some of these older aircraft have been designed or modified to have an AP/FD APR mode that uses lateral and vertical deviation analog signals from the FMS to provide ILS “look-alike” steering commands on the final approach segment of an RNAV approach.

These older generation AP/FD systems may be unable to transition the track change at the FAF within the required tracking parameters, because the AP/FD APR mode is designed to track FMS lateral and vertical deviation signals for straight approach segments. For aircraft equipped with these older generation APFS systems the AP/FD APR mode should only be selected after passing the FAF, once the aircraft has transitioned through the track change and the aircraft is positioned within the required arming and capture parameters.

Prior to the FAF, these older generation AP/FD systems, which cannot use FMS generated pitch steering commands, may require the use of uncoupled vertical AP/FD modes e.g. “vertical speed” (VS), or “pitch” (PIT)) to manually track the vertical flight path. This will require increased pilot workload for the capture and tracking of the vertical path. Any deviation from the vertical path may destabilize the approach or delay the capture of the AP/FD APR mode.

Some FMS manufacturers have coded their FMS’s to prevent them from entering an approach phase (or mode) for these types of approaches. This may not be immediately obvious to flight crews. These FMSs will not provide the correct scaling for the final approach and will not provide the required approach annunciations if not in the approach phase.

Note:

RNAV approaches should not be conducted unless it has been confirmed that the FMS approach phase has been armed and activated.

Some older generation aircraft may have even more rudimentary AP/FD systems to couple to FMS or GPS equipment capable of flying RNAV approaches. These older aircraft may not be equipped with an AP/FD APPR mode to provide ILS look-alike signals on final approach or lack an AP/FD LNAV roll steering capability. These (rudimentary) AP/FD systems may require the use of several AP/FD mode changes including the use of the AP/FD HDG mode to execute the track change at the FAF.

Depending on the aircraft’s AP/FD capability, it may be necessary to fly the approach manually using “raw data” information from the FMS or GPS to achieve acceptable tracking performance. Here again, there is a greater potential for increased pilot workload, less stabilized approaches and degraded situational awareness, because of the lack of automation.

Increased altitude awareness is required for the less capable older generation AP/FD systems, especially with respect to the fix crossing altitudes and minimum altitudes required during the approach.

AP/FD Altitude pre-selections should be carefully made and consistent with the AP/FD vertical mode being used. The altitude preselection for a possible go-around should be established at the appropriate part of the approach.

In summary, the following are potential safety issues associated with RNAV approaches with track changes at the FAF; especially in the case of older less capable legacy AP/FD systems:

1. The increased possibility of an un-stabilized approach and increased pilot workload because of the inability to utilize the AP/FD APR mode, until inside the FAF. This is more critical for those RNAV approaches with a short final approach segment;
2. The increased possibility of an un-stabilized approach and increased pilot workload because of the need to select multiple AP/FD modes in the correct sequence for less capable legacy AP/FD system;
3. The increased need for situational awareness of AP/FD modes selections, altitude constraints and appropriate altitude pre-selections; and
4. The increased possibility of conducting an RNAV approach without the FMS entering the FMS approach mode, if the flight crew is not fully aware of the particular FMS (or GPS) capability.

Recommended action

Transport Canada reminds all pilots, flight dispatchers, air operators, private operators and foreign air operators that they must fully understand and respect the performance capabilities and limitations of the AP/FD and FMS/GPS systems in their aircraft. In particular, the operators of aircraft equipped with less capable AP/FD systems must be aware of:

1. the potential tracking problems associated with RNAV approaches which have a turn at the FAF; and
2. the potential for an unstable approach and excessively high workload, which significantly increases when the FAF is located close to the runway.

Operators should modify or develop procedures to ensure that a “stabilized approach” will be conducted and pilot workload will be minimized. Additional information respecting stabilized approaches can be found in CASA 2015-04, Subject: Stabilized Approach.

Pilots and operators of aircraft with an older generation AP/FD systems must not conduct an RNAV approach with a turn at the FAF, unless they have:

1. confirmed the capability of their AP/FD systems and FMS respecting the subject approaches;
2. established and confirmed the effectiveness of procedures which mitigate any potential safety hazards associated with the subject approaches; and
3. provided adequate training to their flight crews and flight dispatchers (if applicable) respecting the subject approaches.

Operator’s procedures should also specify the criteria which will compel the flight crew to discontinue the approach and initiate a missed approach. These criteria include, but are not necessarily limited to:

1. the altitude(s) where the aircraft must be configured and stabilized during the approach,
2. the minimum altitude(s) for the use of specific AP/FD modes,
3. Activation of FMS/GPS approach mode,
4. undesirable aircraft states, such as inappropriate airspeed and vertical speed, and
5. any other condition(s) or circumstance(s) which would compel the flight crew to conduct a missed approach.

References:

1. NavCanada Aeronautical Information Circular (AIC) 3/18; dated 01 Feb 18
2. Transport Canada CASA 2015-04, Subject: Stabilized Approach.

The following abbreviations are used in this document:

1. AP/FD: autopilot/flight director;
2. APR: “approach” (mode d’un AP/FD);
3. CASA: Civil Aviation Safety Alert;
4. FAF: final approach fix;
5. FAWP: final approach waypoint;
6. FMS: flight management system;
7. FPA: flight path angle (mode d’un AP/FD);
8. ILS: instrument landing system;
9. LP: localizer performance without vertical guidance;
10. LPV: localizer performance with vertical guidance;
11. LNAV: “lateral navigation” (mode d’un AP/FD);
12. MDA: minimum descent altitude;
13. PIT: pitch hold mode (mode d’un AP/FD);
14. SBAS: satellite-based augmentation systems;
15. RNAV: area navigation; and
16. VS: vertical speed (mode d’un AP/FD)

Appendix A

Example of an RNAV approach with a track change (turn) at the FAF/FAWP

Contact office:

For more information concerning this issue, contact a Transport Canada Centre; or contact Inspector Roger Gravelle in Ottawa, by telephone at (613) 991-3426, by fax at (613) 990-6215 or by e-mail at roger.gravelle@tc.gc.ca.

Original signed by

Jean-François Mathieu
for
Robert Sincennes
Director
Standards Branch