1 in 60 rule to find GS without being given distance?

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ktrwcm2014
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1 in 60 rule to find GS without being given distance?

#1 Post by ktrwcm2014 » Sat Dec 02, 2017 11:51 pm

Hey there,

I'm stuck on a problem that requires me to find an approximate groundspeed using1 in 60 rule.

Passing the Chetwynd beacon you wish to check your groundspeed. You tune into the Chetwynd beacon and observe an 8' relative bearing change in 40 seconds. Your approximate groundspeed as can be estimated using the 1 in 60 rule is?
a) 100 knots
b) 80 knots
c) 120 knots
d) 60 kots

the answer is 100 knots but I can't seem to figure out how to come to that answer.
Using the formula for time to station, I got 5minutes to the station but without being given any sort of distance, I'm lost.
The closest answer I've come to was 96 knots but I wasn't sure if I got that number doing the right thing.
Here is how I ended up with the answer 96 kts.
TTS: 40/8= 5minutes
DTS: (TASxTTS)/60
DTS= 8 since we will be 8nm miles away from the beacon if we were to fly with constant heading for 60nm.
Therefore, 8=(TASx5)/60
TAS= 8x60/5=96

Is this the right way of solving the problem above or did I just get lucky with the numbers and ended up somewhere close to the answer 100kts?

Thanks in advance!!
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lownslow
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Re: 1 in 60 rule to find GS without being given distance?

#2 Post by lownslow » Sun Dec 03, 2017 9:25 am

I think distance to the beacon is missing.
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mmm..bacon
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Re: 1 in 60 rule to find GS without being given distance?

#3 Post by mmm..bacon » Sun Dec 03, 2017 9:26 am

Bloody Transport Canada!
Meanwhile, in the real world, what does your GPS say?
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photofly
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Re: 1 in 60 rule to find GS without being given distance?

#4 Post by photofly » Sun Dec 03, 2017 10:44 am

As Lns points out, you need your distance to the station to answer the question as you wrote it.

Also you described an 8’ relative bearing change -that is, 8 minutes of arc, which is two fifteenths of one degree. There are no instruments in aircraft that accurate. Did you mean 8 degree bearing change?

Overall I suspect you transcribed the question wrong.

But, if your speed is 100 knots and you crossed 8 degrees of bearing change in 40 seconds while flying perpendicular to the station you must have been

8 * 60/40 * 100/60 = 20nm abeam the station.

If 8 minutes of arc, then sixty times further.
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ahramin
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Re: 1 in 60 rule to find GS without being given distance?

#5 Post by ahramin » Sun Dec 03, 2017 9:43 pm

I don't get it. Bearing change does not tell you anything unless you know your distance from the beacon or your groundspeed. If you are over the beacon a 180° bearing change takes a couple seconds.
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Re: 1 in 60 rule to find GS without being given distance?

#6 Post by Grey_Wolf » Mon Dec 04, 2017 3:18 am

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Chris M
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Re: 1 in 60 rule to find GS without being given distance?

#7 Post by Chris M » Mon Dec 04, 2017 6:04 am

We're missing at least two pieces of information: Distance to the beacon and relative heading. Assuming you meant 8 degrees change, the rate of change in relative bearing would be fastest if you're passing directly abeam the beacon, and slower the more you're heading towards or away from it.

What a stupid question though... If we're doing all this math I assume we have no GPS in the plane. Fine, we're using paper. Then why not just use times over two points to calculate ground speed? Simple, accurate, fast. The point of a "Rule of Thumb", like the 1 in 60, is to be able to do quick estimates that don't take you away from the important things (like flying the airplane) for too long, and give you a reasonably accurate answer. A method this convoluted is just asking for an extended period of inattentive flying and more likely to result in the wrong answer.

Working backwards I get between 8.333 NM from the beacon, assuming you're passing abeam.

At 100 knots we cover 1.11111 NM in 40s. (100/3600)*40 = 1.11111

Using the 1 in 60 rule, 8 degrees of arc at 60 NM would be 8 miles traveled. To knock that down to 1.1 miles traveled:

(1.111/8)*60 = 8.333 NM.

Makes sense to me, as at 100 knots you'd need to be pretty close to on top of the station to see 12 degrees per minute.
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Re: 1 in 60 rule to find GS without being given distance?

#8 Post by photofly » Mon Dec 04, 2017 2:33 pm

photofly wrote:
Sun Dec 03, 2017 10:44 am
8 * 60/40 * 100/60 = 20nm abeam the station.
Yes, that should have been (40/8*60) * 100 = 8.33 nm.
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