As I explained above, the issue is only that they want to ensure that RNAV routes are retained. The problem arises when “tunnels in the air” mean that all aircraft in the airspace must be RNP, and possibly RF equipped, and their aircraft are therefore debarred.
Timothy wrote:
There is considerable resistance from (mainly) smaller airlines to procedures being introduced for which their aircraft aren’t, and usually can never be, equipped. This is true of RNP in general (because their RNAV is not fault resilient) and RF legs in particular. Every RNP or RF leg procedure comes with a comment from those operating older aircraft saying that there must be conventional or RNAV alternatives which do not add track miles or time penalties.
This is an interesting, rather paranoid, way of looking at things. Actually, it should be the other way around … the conventional or RNAV alternatives don’t add track miles or time penalties, rather RNP (incl RF leg) procedures reduce track miles and time. There is no penalty for using older technology, only a business decision on their part not to invest in the use of new capabilities. The next step is to complain about GPS/RNAV because they feel disadvantaged if they only have VHF nav. They should take responsibility for their own decision making. Claiming sour grapes is just suggesting that they are unable or too poor to compete, so they should be subsidized by the rest. Given how GA pays through the nose just to keep our hobby alive, sorry if I’m not particularly sympathetic.
NCYankee wrote:
AC’s are not regulatory. In general they are “A means” of complying with a subject, but not the “Only means”.
Yes, thanks, that’s what I thought. Someone is suggesting that hand-flown RF legs are “allowed” by AIR (Certification, hence the AFMS) but “prohibited” by AFS (Flight Standards i.e. operations). I can’t see a legal basis for FSS to have a say on that issue for Part-91 operators — obviously they do for Part-121,135 and 91K.
So I understand from the above exchange that the AFMS wording is determinant, which makes sense since it is an FAA-approved document applicable to a specific aircraft (or AML).
bookworm wrote:
To what extent is the Appendix to AC 90-105A binding on a Part-91 operator?
AC’s are not regulatory. In general they are “A means” of complying with a subject, but not the “Only means”.
This is from Chapter 1: Purpose
1.2 Alternative Method. In lieu of following the guidance in this AC without deviation, operators may elect to follow an alternative method, provided the alternative method is found to be acceptable by the Federal Aviation Administration (FAA).
So, Garmin presented an alternate method and it was found to be acceptable by the FAA with the approval of the STC. Note that Garmin did an earlier study for the FAA and recommended that none of this nonsense was required to fly RF leg types and have the pilot reliably meet the 95% navigation criteria. I don’t see any valid reason why RF legs should not be supported with either a roll steering type autopilot or a FD or a EHSI with auto slew. My installation includes all three.
This is from chapter 2: Applicability
2.3 Terms. Mandatory terms used in this AC such as “must” are used only in the sense of ensuring applicability of these particular methods of compliance when the acceptable means of compliance (AMC) described herein are used. This AC does not change, add, or delete regulatory requirements or authorize deviations from regulatory requirements.
IOW “must” is not a regulatory requirement that binds part 91 operators or aircraft manufacturers, if it binds anyone, it is binding on the FAA that if a manufacturer follows the AC with respect to all the “must” requirements, it provides “a means” of compliance, but is never the “only means”.
NCYankee wrote:
I am only aware of two approaches in the US that incorporate RF legs and they are separate charts to the same approach that uses different IAF that do not require the RF leg. If RF is required in an RNAV SID or STAR, RNP 1 is required rather than RNAV 1. I have RF approval on my GTN750 because it is connected to my G500TXi that has auto slew.
I’m in a little debate elsewhere about the FAA position on the use of RF legs without an AP/FD.
AC 90-105A says in Appendix I:
I.2.7 Functional Requirements. RNP procedures with RF legs require the use of an AP or FD
with at least “roll-steering” capability that is driven by the RNP syste. The AP/FD must
operate with suitable accuracy to track the lateral and, as appropriate vertical paths
required by a specific RNP procedure.
I.2.7.1 The aircraft must have an electronic map display depicting the RNP-computed
path of the selected procedure.
I.2.7.2 The flight management computer (FMC), the FD system, and the AP must be
capable of commanding a bank angle up to 30 degrees above 400 feet above
ground level (AGL).
I.2.7.3 The specified bank angles comply with RTCA/DO-236C and are consistent
with a common procedure design criteria that accommodates all aircraft
categories; including those with the highest approach speeds. Aircraft with
lower approach speeds will typically not achieve these bank angles in normal
operations.
However the GTN750 with autoslewing EHSI (G500TXi) will have airworthiness approval for uncoupled (no AP/FD) RF legs. Specifically, the statement in the AFMS “This installation is equipped to support un-coupled RF leg navigation up to RNP 1.0.” is likely to be checked.
To what extent is the Appendix to AC 90-105A binding on a Part-91 operator?
I wonder what the difference is between the above, and flying an RNAV1 STAR to an ILS (which is probably fairly common).
Here is an article about RF legs that is a year and a half old, but contains some interesting perspective on some of the points raised in the discussion above, at least as far as evolution in the US is concerned:
- some ILS already have GNSS requirements beyond the common DME.
- Garmin project demonstrates RF legs can be flown by hand, and makes recommendations to address specific issues in typical GA aircraft.
It would be TERPS. The radius of turn can’t be less than the required obstacle clearance in TERPS. So R = 3 X RNP value. I don’t think the value used in the SID would be permitted under TERPS. PAN-OPS, I don’t know.
NCYankee wrote:
The SID is RNAV 1. The minimum radius is the total primary (2 X RNAV RNP) and secondary obstacle protection (1 X RNAV RNP), which means the minimum radius for an RNAV 1 RF leg is 3 X 1 NM, or 3 NM.
By TERPS or by PANS-OPS? The RF leg in the Zürich chart had a distance of 3.3 NM and the course change was 90°. That means that the radius was (3.3*4)/(pi*2) = 2.1.
There is also a maximum bank angle allowed of 25 degrees, but 18 degrees is the normal. Any required bank angle above 18 degrees needs to be annotated on the chart. If required, the maximum speed the RF leg may be flown can also be limited to achieve a lower bank angle.
I can’t find any requirement in PANS-OPS for annotating bank angles over 18°.
