Has anyone else noticed the Ground Speed indication on this aircraft is very sluggish in operation while taxying.

 

When taxying I normally have Aivlasoft's EFB running for taxiway identification etc. also this displays the aircrafts Ground Speed.  I have noticed during taxying as I speed up the JS41 GS Indicator in the Nav Display lags quite badly behind my actual speed.  Sometimes I could be taxying at 20 kts but the indicator is only showing 6 kts but slowly increasing.

 

Another thing I noticed was when I slowed down to about 10 kts to turn a corner, during the turn the indicator temporarily displayed dashes.  I don't know if this is by design, I haven't seen this problem on other aircraft types so I don't think its a glitch on my computer.

 

Glen

Do you have any problems with ground speed lag whilst in flight?

I'll have to check that one, I don't often look at the GS in flight except when working out the rate of descent and then I assumed it was right.

 

I did another flight today and it lags quite badly on the ground.  Haven't seen this on any other aircraft.

I have noticed during taxying as I speed up the JS41 GS Indicator in the Nav Display lags quite badly behind my actual speed.

 

Remember that your LNAV groundspeed is dependent on a position calculation after the fact. Your groundspeed indication isn't meant to be a speedometer for you to taxi around the airport. It's really meant to show your groundspeed during flight, though it can be used as a supplementary source of information to determine taxi speed (the primary being your eyes/judgment).

 

What do I mean by that? IRS and GPS derived locations and speeds are based on calculations of distances and times in the past. Unless you put something in the tire or axle to judge your speed (as in a car), this is the only way to get your ground speed. In order to do this, the system takes Time A at Point A, and Time B at Point B, and calculates the differences in the distances over the times. In order to run these calculations, both of these points will be behind you, ergo, the calculation will lag behind your actual speed.

Another thing I noticed was when I slowed down to about 10 kts to turn a corner, during the turn the indicator temporarily displayed dashes. I don't know if this is by design, I haven't seen this problem on other aircraft types so I don't think its a glitch on my computer.

 

This is by design. You'll note, a car's speedometer is very similar in that many don't actually display a 0 position. Even if 0 is displayed, you'll note that between 10-20 and 0 is often at a different scale than the rest of the speedometer.

Kyle

 

Your very detailed explanation makes sense, many thanks for taking the time to answer.  I'm a little surprised PMDG modelled this so accurately as its not supposed to be a 'Study' sim, but then again that's PMDG for you  :smile:

 

 

Your very detailed explanation makes sense, many thanks for taking the time to answer.

 

You're welcome!  Glad it made sense.

 

 

Sometimes I could be taxying at 20 kts but the indicator is only showing 6 kts but slowly increasing.

  Also, if you have selected V/L for Nav information, DME and GS will be based on the VOR or ILS you have tuned in.  And, GS from a VOR/DME or ILS/DME becomes less accurate if you are not flying directly from or to the station.  So if you just landed and have the ILS tuned in, it will not display good GS when taxing in an angle and will actually give you those dashes when close to 90 degrees from the station.  The same if you have the field's VOR tuned in (or one close enough to receive).

 

Do a little test.  Fly  directly to or from a VOR/DME from 20 - 30 miles out or more.  Leave the Nav on LNAV.  The GS should be right on as you compare it between the reading on your DME display and the GS on your EHSI.  Then, enter a turn and see how the GS in the DME display begins to change while it remains the same in the EHSI (it may change some as the wind changes and if you do not maintain airspeed in the turn, but not as drastic as in the GS in the DME indicator).