View Full Version : FMS use below MSA?

29th Mar 2009, 22:38
I am currently flying a turboprop with a single FMS unit, our ops manual states that the FMS must not be used for primary navigation below MSA.

Does this mean that the AFCS cannot be coupled with the FMS or just that independent nav equipment must be used to determine position/track?

If we define the pilot as the primary navigator rather than the autopilot, is it legal to program a pseudo-vortac to follow an NDB approach and monitor the aircraft remains within the approach tolerances?
Your opinions appreciated,

CJ Driver
29th Mar 2009, 22:56
Before you start following the FMS outside the parameters set by your ops manual, you might want to do a bit more investigation into the position sources used by the FMS. If it is NOT based on GPS it may become unreliable at low altitudes, and there may therefore be good reason for the limitation in the manual.

29th Mar 2009, 23:19
The FMS has a GPS (with RAIM) as its primary source and VOR/DME, DME/DME with VPU monitoring as secondary.

I accept the limitations of single fit FMS that are not approved to fly approaches, and rightly so.

My question is, does that mean it cannot be coupled to the autopilot?

My opinion is that if I am using the ADF as the primary source of navigation data, what the AFCS is selected to is irrelevant, as long as the aircraft is tracking within the NPA tolerances.

29th Mar 2009, 23:27
I don't see it like that- the FMS is controlling the AP and is using itself as the primary nav aid, therefore shouldn't be coupled as instructed. If you want to fly on ground based aids, then do so, but it says you are not to use the FMS as primary nav aid, despite the fact that you are watching and monitoring what the FMS is making the AP do.

29th Mar 2009, 23:38
I see your point but our manual is not that specific, there is no specific guidence on whether the FMS may be coupled. (simply what nav infomation we may use)

I have witnessed many pilots using the track infomation provided by the FMS whilst following the needles using the heading bug. If they had simply left the aircraft in AUX NAV then their capacity for monitoring the conventional instruments would increase, if the aircraft were to deviate more than 5 degrees then obviously the ADF would be the deciding factor.

30th Mar 2009, 02:38
IMHO the wording may be interpreted as enabling AFCS coupling but requiring the navigational position to be verified with an approved aid.
You, the pilot, control the aircraft either via the AFCS (AP/FD) or with manual flight, this includes navigation. You establish the navigational position of the aircraft whichever nav aid is chosen for ‘primary’ use; – the AFCS does not decide the position or the nav aid’s accuracy.

In practical terms I would strongly advise against coupling to the AFCS, and in situations where there are few if any precision nav aids, do not even display the FMS on EFIS.
The rationale for this is that the use of AP or the EFIS indications might inadvertently distract the crew from the approved nav aid as usually the AP and EFIS are the most prominent cues..
My experience of this was with a long range nav aid subject to drift, not approved for approaches, but still ‘navigating’ during a remote area NDB approach. Both the FMS and ADF were displayed on EFIS; this resulted in high mental workload to ignore the ‘dominant’ FMS symbols. The AP was used in the hdg mode; the FMS was incorrect by at least 2nm – only verifiable when visual.

Pontius's Copilot
30th Mar 2009, 10:28
Chris, your profile says your current type is DHC8, that being the case - either Q300 or Q400 series - you'll be aware of the capabilities of that type and its nav kit. Your turboprop type's single FMS installation must be certified for B-RNAV operation (more likely it is already P-RNAV certified), it is very unlikely to be a single-source FMS (ie, it will prioritise GNSS/GPS, but will also receive DME and perhaps VOR info).

'Map Shift' error only happens to single-source RNAV equipment, most systems will 'flag' system issues and position uncertainty messages. 'RAIM' is only relevant for GPS-dependent procedures, ie where the capability to fly a given procedure depends on the availability of valid GPS signals (principally, RNAV/RNP approaches). You need to get hold of and study the Handbook/User Manual for your FMS.

Any B-RNAV equipped/approved aircraft may be flown using the FMS for steering guidance above MSA. European aviation authorities (incl. UK CAA) accept that (even public transport) operators fly conventional departures (from take-off) and arrivals using the FMS ... PROVIDED the procedure is designed to be flown using VOR/DME/NDB aids, and those aids are used to confirm FMS track-keeping accuracy. It must be understood that the conventional navaid information will have priority over FMS information if any conflict arises. It is not permitted (as a B-RNAV operator) to use RNAV to fly departures and arrivals that cannot be flown solely using conventional navaids, or that are labelled RNAV Only.

Subject to the above, no reason at all why you should not use the autopilot coupled to FMS for steering the aircraft, this will give you the capacity to monitor conventional navaids fully and revert to heading or VOR coupled nav if necessary.

Try this for good straightforward guidance -

31st Mar 2009, 16:32
In answer to your original question:

"...our ops manual states that the FMS must not be used for primary navigation below MSA."

Let me follow the logic - descent in IMC below MSA is conducted via an instrument approach and therefore requires navaids designed, approved and flown for that purpose. I am guessing that either the FMS equipment, the installation or you are not approved to conduct FMS-based approaches. So the regulator and your employer don't want you to do it, so they ban it in the Ops Manual.

But you don't accept that, so you are looking for some form of justification for violating the ban - correct?

"Does this mean that the AFCS cannot be coupled with the FMS or just that independent nav equipment must be used to determine position/track?"

As a purist, the answer is that the FMS should not be coupled to the autopilot below MSA and you should be navigating by either coupling to a suitable navaid or steering by reference to that aid. The FMS data should be monitored by the PNF/PM as part of the available data to back up the primary tracking conducted by the PF in accordance with the terrestrial navaid information.

In practice, children of the magenta pretend to monitor the terrestrial navaids while letting the FMS drive them to their destination, intended or otherwise. And they have been known to miss detecting failures of the navaid or the airborne receivers while "monitoring", as well as failing to detect waypoint and track selection errors, let alone FMS accuracy issues.

"If we define the pilot as the primary navigator rather than the autopilot, is it legal to program a pseudo-vortac to follow an NDB approach and monitor the aircraft remains within the approach tolerances?"

Whatever you choose to define as the primary navigator becomes a little irrelevant - in the end it will be the Judge who decides what actually took place and the extent of your culpability, including a few words about the difference between "reckless" and mere "negligence"!

There are invariably good reasons why the Ops Manual has bans on certain practices and why most jurisdictions have a rule requiring you to comply with your Ops Manual or face a criminal sanction.

Stay Alive,

31st Mar 2009, 17:05
Pontius's Copilot, take care with the generalised statement “'Map Shift' error only happens to single-source RNAV equipment, …” as in many situations this can be incorrect. From you own example of an FMS with GPS/DME input; when flying in an area without DME or poorly located DMEs, a degraded GPS can result in the FMS position ‘slipping’ and the accuracy being less than required for an NPA.

Incident 3 in "Celebrating TAWS ‘Saves’: But lessons still to be learnt (http://www.icao.int/fsix/_Library%5CTAWS%20Saves%20plus%20add.pdf)" illustrates a hazard in programming FMS approaches negating safety monitoring, as well as presenting a confused navigation display.