Nav/App modes on autopilots

[nad]

Hi all

May be obvious but can someone tell me if there is any difference in using Approach Mode or Nav Mode when intercepting and then tracking a VOR radial on an instrument approach. I appreciate the differences when on an ILS with APP as it will capture the G/S as well. Does it try to maintain it to tighter tolerances maybe?

Cheers

[happybiker]

Hi Nad

From the notes I have, the VOR/LOC functions after mode capture are almost identical. Bank angle and roll rate limits for VOR/LOC are the same after capture. The notes show a radio rate signal which is used in LOC captured mode but not always used after VOR capture. The radio rate signal functions in a similar manner to the course error signal after capture in maintaining the aircraft on the beam centre line.

[nad]

Thanks HB for your help.

Nad

[Intruder]

Depends on the airplane and the Autopilot. In the 744, Approach Mode is only used for LOC and ILS approaches.

[ReverseFlight]

Brings to mind an infamous example of an air accident involving NAV instead of APP mode : the Gulfstream which was headheading to pick up President Bush Jr crashed on approach at Houston in 2004:
Accident Investigations - NTSB - National Transportation Safety Board (http://www.ntsb.gov/investigations/fulltext/AAB0606.html)

[Denti]

@intruder, I believe in the 748 the approach mode is used for non precision approaches as well, same as in the 787 and 737NG. Apparently it depends not only on type but on variant as well.

[dixi188]

Just a comment about aircraft type numbers.

The last post mentions "748" meaning "747-8". I find this confusing as "748" has previously always related to the HS 748.

Perhaps there is a need for a preferred way of referring to aircraft types.

Also there are many posts on here that don't mention the type and a little detective work can be needed to get the gist of the thread.

[Dan Winterland]

Think this might be an Airbus question.

If it is, the answer is for lateral Nav, no - there will be no difference. The FMGC will fly the inbound track of the VOR from the FMGC derived track and you will fly inbound as accurately as if you had pressed the APP button. You can of course fly the vertical profile controlling it with either Vertical Speed or Flight Path Angle selected. What pressing the APP button will do is fly the 3D profile of the approach. The FMAs will annuciate FINAL APP and fly the vertical profile as well.

To do this, the procedure has to be an RNAV overlay approach and the operator has to be cleard to do this type of approach. Not all are.

[Intruder]

If a question is airplane or engine specific, then the poster should make that clear.

[Capn Bloggs]

If a question is airplane or engine specific, then the poster should make that clear.
Flight Simmer? ;)

[Pub User]

Just to refer to the original question:

is any difference in using Approach Mode or Nav Mode when intercepting and then tracking a VOR radial on an instrument approach

Yes there is a difference. In approach mode it will track the VOR (as accurately as it can), but in NAV mode it will follow a calculated track, based on the known VOR position, using its normal navigational mode, which varies from type to type (and variant) but nowadays is normally heavily GPS biased.

If the approach involves flying directly over the VOR, which is often the case, then it is generally preferable to use NAV mode as it will not then involve negotiating the 'cone of confusion' over the beacon.

[Gen. Anaesthetic]

Dash 8 Q400:

There are 2 navigation modes:
1/ "Blue needles", which is a colloquial term for basic navigation. Flight Guidance Computer (FGC) uses NAV/APP/BC mode and tracks via VOR/LOC/ILS input. No GPS input is involved. It's as if you are flying in your old 172 with steam driven gauges. The limitation is that in APP mode, there must be a DME associated with the VOR and in APP mode there are tighter steering tolerances. As a general rule it is not uncommon to see the aircraft wandering a little when in VOR mode compared to APP mode.
2/ FMS: In this mode the flight guidance computer is taking navigation input from the FMS. Depending on the software installation for the FMS, the FMS allows VOR, RNAV, and NDB approaches but not ILS approaches. In all cases LNAV is the navigation mode used so APP mode is never used. That said, the FMS itself has approach mode, but I won't go into that as it might just add to confusion! Normally, the FMS uses input from all sensors (ground and space based), but for a VOR approach it inhibits GPS input and uses only VOR input from the ground based aid on which the approach is based.

In practice we only use blue needles for ILS approaches and for all others we use FMS mode. Using FMS allows vertical gates to be used via VNAV, whereas in blue needles it is up to you to set vertical speed to not bust altitude limits throughout the approach. Only exception might be the rare situation where we have to do an NDB approach and the approach is not loaded in the FMS database.

For VOR and NDB approaches conducted in FMS mode, the pilot not flying selects their MFD (the navigation display) to blue needles so that the ground based aid input can be easily monitored.

[Denti]

@PubUser, that might be a correct description for one airplane or variant, but certainly not for all. On "my" type when flying a VOR approach in approach mode the autopilot or flight director will be following a database derived final approach course and glide path. Actually the same thing as using LNAV/VNAV, just with the added bonus that there are active warning calls and the procedure is identical to a normal ILS. VORLOC would follow the radial and leave the vertical path to the pilot, however that procedure is not used anymore.