With the old HSI/EHSI system, your course pointer is turned manually to the next track at each waypoint, or with the EHSI it automatically flips to it, and the plane turns then, or about then.
This works fine enroute, but it doesn't fly an accurate intercept on sharp turns (say 90 degrees) except by coincidence, and even that will work only at a specific groundspeed (140kt for my kit, for a 90 degree turn).
With GPSS (also called roll steering) the GPS transmits a digital data stream (ARINC429) which contains a roll angle, which the GPS adjusts according to where in the turn you are. I think it starts at something like 20-25 degrees and as you approach the target track intercept it reduces the figure, to zero as you reach it. It's crude but it works. With autopilots which don't accept the ARINC429 data directly (all the old ones) you buy a roll steering converter such as the GDC31 which takes in the ARINC429 data and fakes a heading bug signal and the autopilot is used in the HDG mode.
But none of these systems seem to take in airdata e.g. the wind vector. So if you are flying a sharp track change, say 130 degrees, and you have a massive tailwind, it will compute the turn using the current GS but it won't know that your GS will change massively as you turn.
Do any of the new system compute the turn exactly, using wind data?
Hi Peter.
I think Avidyne R9 does:-
FMS Vectors™ - Fly the Magenta Line All The Time The FMS900w provides an easier way to fly with Avidyne’s innovative new FMS Vectors feature. With FMS Vectors capability, the FMS900W calculates the transition from the en route GPS phase of flight to the ILS intercept, draws a curved intercept flight path on the display and provides wind-corrected roll steering commands to the autopilot. In previous-generation navigators, the pilot must often artificially suspend navigation between an en route GPS-guided course and the terminal phase of flight to a VHF navaid such as an ILS.
(my bold)
As far as I know, the GPSS output from the Garmin GTN series does this if the unit is fed an the necessary data on the ARINC bus.
I've seen it working on a TBM with a G600/GTN750 setup, the magenta line clearly allows for wind, and there is almost no hunting for a stable heading after turning onto a new track - it just rolls out onto a wind corrected heading.
Now, if only I had an autopilot to connect to my 650...
The "old" 430w / 530w takes wind into account during a hold (and provides steering command as well)
http://peter-ftp.co.uk/screenshots/2017-05-03_170231.jpg
I'm not sure if it works during regular track interception but I don't see why it wouldn't.
Many thanks. Very interesting.
That blog actually seems to refer to the non-W 430/530 - from 2007.
I asked about this several times and the answer was always... no the airdata is not used for anything other than displaying the wind vector.
I even asked the people who make and sell airdata computers (DAC, etc). I also asked a number of owners of an ADC (e.g. the popular ADC200) and the answer was the same. Yet, these people paid several thousand € to have one installed!
When doing a holding pattern with the GNS 530/430 you will see a wind corrected holding track instead of the normal oval shape. This will make you look you look like a pro because every holding pattern will be perfect
That is very good, but one cannot fly a hold with both Rate 1 turns if there is any crosswind. What does it do? It must either make the hold bigger (so one turn is flown Rate 1 and the other with less than Rate 1) or the other way round (exceeds Rate 1 on one of the two turns).
In my setup the 430W supplies the roll steering information and the Aspen does the GPSS conversion for the autopilot which is then sent to the AP. I am pretty sure the Aspen takes the wind vector into account, otherwise it would be rather pathetic for low speed strong wind operations and I have never seen it make a lot of corrections after completing the turn. I now have a new autopilot (DFC90) which is all digital and does GPSS by itself and I couldn't test it so far.
When doing a holding pattern with the GNS 530/430 you will see a wind corrected holding track instead of the normal oval shape. This will make you look you look like a pro because every holding pattern will be perfect.
For my IR checkride, I had to get a wind estimate before the flight and then do heading and time corrections for the holding based on some weird formulae that take the wind vector as input (I forgot them right after the exam). Obviously the holding was crap but the examiner was happy I knew the official formula and flew it accordingly.
Peter,
I don't think the GNS/GTN series roll steering bank angle is as crude as you think for course changes, even the 90 degree ones. Even without air data, it adjusts the bank angle as required based on the track and groundspeed. The WAAS units have position updated at 5 Hz, but even the non WAAS units do a very good job. Of course there can be some overshoot if the ground speed is too high and the autopilot can't comply with the bank angle requirement for the turn. Until today, my Bonanza has not had air data available. I am picking it up today from the avionics shop where it has had an air data unit added along with my new ADSB system. I added the air data to pickup heading input to my GNS530W so it would display traffic on the Map page. My configuration is a KCS-55A with boostrap, a Shadin ADC-200, a GNS530W, a GDL88 ADSB UAT Out, dual frequency In, and a STec 60-2 with GPSS.
It's normal on small jets and upwards.
Peter wrote:
One experienced bizjet pilot informs me that jets with an airdata computer do it, but some do it better than others, which to me suggests they don’t pre-calculate the turn exactly.
Those with an ADC that feeds into the FMS do. ie any modern bizjet.
