Question about Ep 7: Waypoint Navigation video

Hey guys,

I just watched Wags' video on Waypoint Navigation and had a question.

Towards the end of the video, Wags walks through how the Time-on-Target functionality works. In order to set it up, he inputs the Ground Speed, desired Time on Target, and the reference waypoint to serve as the "target".

What wasn't clear to me was why Ground Speed needs to be input? If you have a target location and a time you want to get there, surely the desired ground speed is the factor that's calculated by the onboard computers? That seems consistent with the arrow under your IAS that tells you whether you need to speed up / slow down in order to arrive on time.

I'm sure I'm missing something really obvious here, but I'm not sure what.

Any insights?

My experience with CAS was not extensive (marine pilots forte not navy) but my understanding was you needed to give the computer an idea of what speed you wanted to maintain enroute to the target else the computers could have you flying at 200 kts over the target which would not be good for all kinds of reasons. And flying 200 kts IAS could be 270 or 150 kts GS depending on the winds in the target area.

Remember you had weapon envelopes to consider for CAS.

Thanks Neofightr.

The logic of that explanation definitely makes sense, but I still can't wrap my head around how that would work in practice. How would the FCC resolve the disparity if I set a desired ground speed of 500 kts, but set a desired TOT that would require me to fly at 200 kts? If the FCC tells me to fly at 500 kts, I'll arrive super-early. If it tells me to fly at 200kts, I'll get there on time, but at the expense of completely ignoring my 500kts desired ground speed.

Something has to give in that scenario, but I'm not sure what.

Just want to follow this discussion for some education.

Thanks Neofightr.

 

The logic of that explanation definitely makes sense, but I still can't wrap my head around how that would work in practice. How would the FCC resolve the disparity if I set a desired ground speed of 500 kts, but set a desired TOT that would require me to fly at 200 kts? If the FCC tells me to fly at 500 kts, I'll arrive super-early. If it tells me to fly at 200kts, I'll get there on time, but at the expense of completely ignoring my 500kts desired ground speed.

 

Something has to give in that scenario, but I'm not sure what.

It doesn't work that way. It boils down to the last two waypoints. The initial point and the target wpt. By inputing said params you are allowing the computer to calculate the exact push time from your ip. You will see the computer tell you the exact time you should leave the ip to get to the target at the TOT with the desired ground speed.

It doesn't work that way. It boils down to the last two waypoints. The initial point and the target wpt. By inputing said params you are allowing the computer to calculate the exact push time from your ip. You will see the computer tell you the exact time you should leave the ip to get to the target at the TOT with the desired ground speed.

As someone who is relatively new to all of this, if GS is a constant, and airspeed is a variable, does the computer automatically do the necessary calculations to put you at the target on time?

It doesn't work that way. It boils down to the last two waypoints. The initial point and the target wpt. By inputing said params you are allowing the computer to calculate the exact push time from your ip. You will see the computer tell you the exact time you should leave the ip to get to the target at the TOT with the desired ground speed.

Ah! THAT'S what I was missing. Thanks!

So in Wags' example from the video where he set...

• Ground Speed - 500 kts
  
• Target - WP 6
  
• TOT - 14:20:00Z
  

...I assume that the FCC would automatically treat WP 5 as the IP, right? The pilot would then loiter (if needed) at WP 5 until the push time, after which the FCC would direct the pilot to reach WP 6 at 14:20:00Z at around 500 kts ground speed.

Is that correct?

As someone who is relatively new to all of this, if GS is a constant, and airspeed is a variable, does the computer automatically do the necessary calculations to put you at the target on time?

Yes. The FCC will constantly recalculate your required airspeed, and provide constant feedback via the arrow beneath the IAS readout on the HUD.

Ah! THAT'S what I was missing. Thanks!

 

So in Wags' example from the video where he set...

 

• Ground Speed - 500 kts
  
• Target - WP 6
  
• TOT - 14:20:00Z
  

...I assume that the FCC would automatically treat WP 5 as the IP, right? The pilot would then loiter (if needed) at WP 5 until the push time, after which the FCC would direct the pilot to reach WP 6 at 14:20:00Z at around 500 kts ground speed.

 

 

Is that correct?

Yes. I can't remember if you can designate an ip but I think it assumes the previous wp to the target wpt is the ip.

From the RAAF FA18A Flight Manual: