Finding Aircraft Groundspeed (G/S) and Track
Thread Starter
Join Date: Aug 2013
Location: Ireland
Age: 32
Posts: 48
Likes: 0
Received 0 Likes
on
0 Posts
Finding Aircraft Groundspeed (G/S) and Track
Hey guys. I was wondering if anyone would be able to answer the following question in simple terms please. I have drawn the graph roughly to scale but cannot get an answer? The question is as follows:
Find the aircraft G/S and track given that the aircrafts true airspeed was 75 knots and its heading 130 Degrees and wind velocity 225 Degrees/25kts?
Many thanks.
Find the aircraft G/S and track given that the aircrafts true airspeed was 75 knots and its heading 130 Degrees and wind velocity 225 Degrees/25kts?
Many thanks.
Meikleour
I might be missing something but as I read it I agree with you. The question gives a fixed heading and asks you to find the resultant track given the wind, not find the heading to fly a given track.
Ballpark figures for what it's worth, but Max drift is 20 degrees as previously mentioned..you do indeed get max drift because it's pretty much a beam wind with (from the right, with 5 degrees of tailwind component), so the resultant track is going to be about 110 degrees.
FWIW since you've also got a smidgen of tailwind the ground speed is going to be slightly >75 knots
I might be missing something but as I read it I agree with you. The question gives a fixed heading and asks you to find the resultant track given the wind, not find the heading to fly a given track.
Ballpark figures for what it's worth, but Max drift is 20 degrees as previously mentioned..you do indeed get max drift because it's pretty much a beam wind with (from the right, with 5 degrees of tailwind component), so the resultant track is going to be about 110 degrees.
FWIW since you've also got a smidgen of tailwind the ground speed is going to be slightly >75 knots
Last edited by wiggy; 14th Jun 2017 at 08:58.
Thread Starter
Join Date: Aug 2013
Location: Ireland
Age: 32
Posts: 48
Likes: 0
Received 0 Likes
on
0 Posts
Thank you
Thank you all very much for your replies. Wiggy I do think you are correct but can you go through your calculations e.g max drift of 20 Degrees etc and detail exactly how you got that answer? I am a total beginner to all of this so please forgive my lack of knowledge everyone. Also, do pilots have to do this for every flight in their heads (or on paper) or is it all on computer now and is this a typical question in the ATPL exams?
Join Date: Jan 2007
Location: Weston Super Mare/UAE
Age: 60
Posts: 406
Received 0 Likes
on
0 Posts
Gally2, wiggy mentions max drift. There is a rule of thumb for this, which is windspeed/TAS in nm/ min. Ie 25 divided by 5/4 (75 in nm/min). Dividing by 5/4 is the same as multiplying by 4/5......4/5 times 25 is 20. Thus, for a 75 TAS, the drift cannot exceed 20 degrees. The trick is knowing how much of that max drift to apply. In your example it is easy since the wind is 95 degrees off your nose or 85 off of your tail. We thus use the max drift. So, heading 130 degrees with a 20 degree left drift gives you a track over the ground of 110 degrees.
The way to evaluate how much of this max drift is to look at the wind angle, ie the angle that the wind makes with the longitudinal axis of the aircraft. We use a clock code.....if the wind angle is 15 then we use 1/4 max drift...20 and we use 1/3. 30 equates to a half, 45 is 3/4 and 60 and above is max drift. In other words, we take the wind angle and liken it to minutes on your watch. Hence 30 degrees is 30 minutes, ie a half hour and thus half max drift.....45 degrees is 45 minutes, ie 3/4 of an hour so use 3/4 max drift. Get the idea?
The wind angle is actually the shorter angle between the wind and the longitudinal axis....thus, in your example is actually 85 degrees behind you....hence max drift. For the groundspeed, we use (90-wind angle) and then use the same analogy with the watch face. Thus (90-85) is 5. 5 minutes is 1/12 so, as the wind is marginally behind you then we add 1/12 of 25 kts to your airspeed to get the groundspeed. Thus, in my head, I would tell you that your groundspeed is 77 kts. In reality, for nav, I call that 75 and thus your eta is 4/5 of your distance to go, since you are effectively doing 5/4 miles per minute.
So, to answer your question, my head tells me that your track 110 at a groundspeed of 77 kts. The CRP5 will finesse this but I will be pretty damn close.....
Back to my 380....
The way to evaluate how much of this max drift is to look at the wind angle, ie the angle that the wind makes with the longitudinal axis of the aircraft. We use a clock code.....if the wind angle is 15 then we use 1/4 max drift...20 and we use 1/3. 30 equates to a half, 45 is 3/4 and 60 and above is max drift. In other words, we take the wind angle and liken it to minutes on your watch. Hence 30 degrees is 30 minutes, ie a half hour and thus half max drift.....45 degrees is 45 minutes, ie 3/4 of an hour so use 3/4 max drift. Get the idea?
The wind angle is actually the shorter angle between the wind and the longitudinal axis....thus, in your example is actually 85 degrees behind you....hence max drift. For the groundspeed, we use (90-wind angle) and then use the same analogy with the watch face. Thus (90-85) is 5. 5 minutes is 1/12 so, as the wind is marginally behind you then we add 1/12 of 25 kts to your airspeed to get the groundspeed. Thus, in my head, I would tell you that your groundspeed is 77 kts. In reality, for nav, I call that 75 and thus your eta is 4/5 of your distance to go, since you are effectively doing 5/4 miles per minute.
So, to answer your question, my head tells me that your track 110 at a groundspeed of 77 kts. The CRP5 will finesse this but I will be pretty damn close.....
Back to my 380....
Gally2
Used to be taught in the military as part of pilot nav when you sometimes had to plan or replan something with minimum time/kit, TBH I can't remember where it sits in the civvie syllabus/ATPL, but the same maths certainly has it's uses in the civvie world (it's rare these days but e.g. flying "raw data"/"non computer" approaches where you need to work out a heading to fly a published track towards or away from a beacon or down an ILS, working out drift corrections for holding patterns, etc.).
Just to amplify slightly on the answer you got: "Max drift" is literally the maximum drift that a given wind can produce ( i..e when it is on the beam), It's calculated by the formula captainsmiffy describes (windspeed/TAS in miles per minute), and then for a wind that is not "on the beam" is corrected to give actual drift using the described "clock code".
captainsmiffy - Thank you very kindly for offering the full explanation......I'm rather glad you saved me the hard work and possible embarrasment - my "A2" ticket is well past it's sell by date....
Also, do pilots have to do this for every flight in their heads (or on paper) or is it all on computer now
Just to amplify slightly on the answer you got: "Max drift" is literally the maximum drift that a given wind can produce ( i..e when it is on the beam), It's calculated by the formula captainsmiffy describes (windspeed/TAS in miles per minute), and then for a wind that is not "on the beam" is corrected to give actual drift using the described "clock code".
captainsmiffy - Thank you very kindly for offering the full explanation......I'm rather glad you saved me the hard work and possible embarrasment - my "A2" ticket is well past it's sell by date....
Last edited by wiggy; 15th Jun 2017 at 08:33.
Join Date: Jan 2007
Location: Weston Super Mare/UAE
Age: 60
Posts: 406
Received 0 Likes
on
0 Posts
Pleasure, wiggy.....i apologise for any errors and/or omissions as it was written rather late following my delivering the UPRT sim and with a glass of red in me!
Hope that it all made as much sense as I thought it did at the time......!
Hope that it all made as much sense as I thought it did at the time......!
