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# How far will an aircraft travel in 2-1/2 minutes with a groundspeed of 98 knots?

Private Flying LAA/BMAA/BGA/BPA The sheer pleasure of flight.

# How far will an aircraft travel in 2-1/2 minutes with a groundspeed of 98 knots?

10th Apr 2015, 17:37
#21 (permalink)

Join Date: Jan 2004
Location: West Coast Canada
Posts: 3,666
To get an exact answer for a test, posters have already showed the correct formula.

For in flight use what you really want is TLAR (That Looks About Right) skills.

This means using easy approximations to get a "good enough" number.

So in for your original question "how far will I go at 98 kts in 2 1/2 mins" I would do the quick mental calculation as follows.

-Round up or down to the nearest 1/2 mile per min ( ie 60 kts = 1 mile/min, 90 kts = 1 1/2 mile/min 120 kts = 2 mile/min. Therefore 98 kts is closer to 90 than 120 so use 1 1/2 mile/min value

- calculate the whole numbers first than add the remainder. so 2 times 1.5 = 3 miles plus half of 1.5 ( for the remaining 1/2 min )= 3/4 gives you 3 + 3/4 = 4 1/4 miles an answer that is close enough.

You can do this in your head in maybe 2 seconds. For in flight calculations it will be rare that a calculation of this kind will not be good enough. If you need more accuracy you should use a Whiz Wheel but again a mental TLAR calculation is a good check.
10th Apr 2015, 18:19
#22 (permalink)

Join Date: Sep 2006
Location: Scotland
Age: 80
Posts: 1,435
Precision: Shirley 98 divided by 60 = 1.633333333333 per minute, times 2.5 minutes = 4.08333333333. Or have I missed something?
The difference between that and my original "about 4" is, you will travel a further 506.666464 yards, provided there are 6080yards in a nautical mile?
If the answer governs the decision to go for a landing at home base or not because in 2.5minutes it will be 30mins after sunset and you will be illegal, so a landing at a closer airfield may be required, then absolute precision may be necessary. If this is to pass an exam question then pick the closest of the multiple choices, if it is a general practical Nav question then by the time you have figured it out you will be there already. If you read all my crap first you will be well past it and the whole thing will be irreverent.
10th Apr 2015, 19:29
#23 (permalink)

Join Date: Jun 2002
Location: CYYC (Calgary)
Posts: 5,297
Hey Crash One, I think you meant 'feet' not 'yards'.

Which is a nice segue into an issue which has bedeviled my business, oil exploration. Depending where I am in the world, the units can be SI, or Imperial or a mixture! Then on top of that, historical documents can be in different units.

Some time ago, a colleague was having a problem with data from a document showing the depths to geological formations ("tops") in a exploration well and asked me to take a look. The numbers did not pass the BPF's TLAR test.

"Are these tops about three times bigger than you were expecting?"
"Yes. Why?"
"You've got a feet to metres problem."
10th Apr 2015, 21:19
#24 (permalink)

Join Date: May 2007
Location: Dark side of the Moon
Posts: 313
For me, 2.5 minutes is 1/24th of an hour (5 minutes is 1/12th), which is very close to 1/25th, just as 98 Kts is damn close to 100.

So 1/25th of 100 is 4 NM, near as dammit the answer.

FBW
10th Apr 2015, 21:26
#25 (permalink)

Join Date: Sep 2006
Location: Scotland
Age: 80
Posts: 1,435
My mistake, no wonder my navigation is bluddy rubbish
10th Apr 2015, 22:22
#26 (permalink)

Join Date: Nov 2000
Location: Cambridge, England, EU
Posts: 3,435
So in for your original question "how far will I go at 98 kts in 2 1/2 mins" I would do the quick mental calculation as follows.
I'm still trying to work out the real world case for wanting to do such a sum.

I can see it the other way around - "at 98 knots how long will it take to go four miles" is part of answering the "report your estimate for the beacon" question.
10th Apr 2015, 22:37
#27 (permalink)

Join Date: Apr 1999
Location: In front of a computer
Posts: 1,928
As usual we are looking at the question from the wrong end of the telescope.

Simply slow down to a G/S of 60kts - enjoy the view - and have no further problems with mental maths
11th Apr 2015, 00:13
#28 (permalink)

Join Date: Sep 2006
Location: 23, Railway Cuttings, East Cheam
Age: 64
Posts: 3,121
Hey Crash One, I think you meant 'feet' not 'yards'.

Which is a nice segue into an issue which has bedeviled my business, oil exploration. Depending where I am in the world, the units can be SI, or Imperial or a mixture! Then on top of that, historical documents can be in different units.

Some time ago, a colleague was having a problem with data from a document showing the depths to geological formations ("tops") in a exploration well and asked me to take a look. The numbers did not pass the BPF's TLAR test.

"Are these tops about three times bigger than you were expecting?"
"Yes. Why?"
"You've got a feet to metres problem."
Wasn't there a space probe to Mars that went spectacularly tits up because someone used imperial measurements instead of metric?
11th Apr 2015, 01:12
#29 (permalink)

Join Date: Dec 2013
Location: Paso Robles
Posts: 261
I appreciate the help. As Keith said I've just begun my PPL.
Forget the PPL, this is a high school type question.
11th Apr 2015, 04:49
#30 (permalink)

Join Date: Jun 1996
Location: Check with Ops
Posts: 742
I'm missing some crucial information: is the aircraft going in circles or in a straight line?
No you're not. The aircraft will travel 4.08 nms whether it's flown in circles or not. The question did not ask how much closer to a particular point the aircraft would be after 2.5 mins at a G/S of 98 kts. Since the question mentioned groundspeed specifically then it matters not in which direction(s) the aircraft was flown.
11th Apr 2015, 10:40
#31 (permalink)

Join Date: Jan 2008
Location: lancs.UK
Age: 73
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Nobody has yet asked,-"are you carrying a coconut"
exit to strains of "the white cliffs of dover"

some folks here......no sense of humour
11th Apr 2015, 15:47
#32 (permalink)

Join Date: Jun 2002
Location: CYYC (Calgary)
Posts: 5,297
Wasn't there a space probe to Mars that went spectacularly tits up because someone used imperial measurements instead of metric?
Yes, the Mars Climate Orbiter which entered the Martian atmosphere on the wrong trajectory and burnt up, due to a major units error.

The primary cause of this discrepancy was that one piece of ground software supplied by Lockheed Martin produced results in a United States customary unit ("American"), contrary to its Software Interface Specification (SIS), while a second system, supplied by NASA, that used those results expected them to be in metric units, in accord with the SIS. Software that calculated the total impulse produced by thruster firings calculated results in pound-seconds. The trajectory calculation used these results to correct the predicted position of the spacecraft for the effects of thruster firings. This software expected its inputs to be in newton-seconds.
12th Apr 2015, 06:09
#33 (permalink)

Join Date: Mar 2014
Location: The World
Posts: 1,234
Steve, we are armed coconutians and You don't frighten us, English pig-dogs! Go and boil your bottoms, sons of a silly person. I blow my nose at you, so-called Arthur-king, you and all your silly English kaniggets. Thppppt! [THE GUARD] Wasn't there a discussion on a swallow and a one pounds coconut with regards to the groundspeed question, but if I remember correctly they argued on airspeed for the swallow, not groundspeed? Now remains the question how to get the 43 times per seconds wings beat on a Cessna? Ok, ok, ok, weather is brilliant and I'm better off in the cockpit.
12th Apr 2015, 10:43
#34 (permalink)

Join Date: Sep 2006
Location: Scotland
Age: 80
Posts: 1,435
Wasn't there some other piece of expensive hardware that got its feet tangled up with metres and looking for a mountain to align with wandered off trying to find a planet with a 25000 metre high mountain?
13th Apr 2015, 08:06
#35 (permalink)

Join Date: Feb 2009
Location: Hampshire
Age: 67
Posts: 113
7.56km (for some reason glider pilots calculate distance in km, height in ft, speed in knts and weight in pounds or kg! Confusing)
13th Apr 2015, 10:14
#36 (permalink)

Join Date: Sep 2006
Location: Scotland
Age: 80
Posts: 1,435
Visibility in metres, Nav distances in nautical miles, heights in feet, Nav tracks in deg true, wind in deg magnetic, fuel in litres, imp gallons, US gallons, pounds, or kilograms, weights in pounds or kilograms, No wonder we get confused, wasn't there a guy called British Standards?
13th Apr 2015, 10:33
#37 (permalink)

Join Date: Aug 2000
Location: uk
Posts: 1,644
Working as a chemist in industry one learned very quickly to change from g/litre, oz./gall (Imperial), oz./gall US, degrees Centigrade and Fahrenheit.

I also received some good lessons in the use of precise terms such as the difference between 2 1/2 and 2.5. I once went to the Toolroom foreman, a God in the motor industry in the 60's, and asked for a strip of steel 0.5 in. wide.

He asked if I really wanted it 0.5 in. wide and I said yes, half an inch. "But you've written 0.5 in.", he said rather scornfully. "Yes, I said, Half an inch" .

Eventually, he patiently explained to me the error of my ways, a lesson I have never forgotten.

I also learned the difference between precision and accuracy and was amazed how many professional technicians didn't. This became embarrassingly obvious when the calculator and digital instrumentation became readily available.
13th Apr 2015, 11:19
#38 (permalink)

Join Date: Sep 2006
Location: 23, Railway Cuttings, East Cheam
Age: 64
Posts: 3,121
This isn't directed at the OP who is studying for his PPL and must do it correctly but I'm always a little bemused when people argue the toss between 100 kts and a 101 kts when discussing GA flying. A very short while after my license issue and a few hours flying in the real world I quickly came to the conclusion that everything in GA is plus or minus 5.

I took some friends flying to North Yorkshire the other day, neither of them experienced in GA flying. The usual questions of 'How high and how fast will we go' were met with '130kts and 3,000 feet.' It was very thermic over the moors and as the ASI went between 125-135 kts and the altitude did it's best to shove me around while I was trying to keep within a couple of degrees of my planned course and failing I was thinking 'It's all plus or minus 10 actually'.
13th Apr 2015, 16:55
#39 (permalink)

Join Date: Sep 2006
Location: Scotland
Age: 80
Posts: 1,435
Working as a chemist in industry one learned very quickly to change from g/litre, oz./gall (Imperial), oz./gall US, degrees Centigrade and Fahrenheit.

I also received some good lessons in the use of precise terms such as the difference between 2 1/2 and 2.5. I once went to the Toolroom foreman, a God in the motor industry in the 60's, and asked for a strip of steel 0.5 in. wide.

He asked if I really wanted it 0.5 in. wide and I said yes, half an inch. "But you've written 0.5 in.", he said rather scornfully. "Yes, I said, Half an inch" .

Eventually, he patiently explained to me the error of my ways, a lesson I have never forgotten.

I also learned the difference between precision and accuracy and was amazed how many professional technicians didn't. This became embarrassingly obvious when the calculator and digital instrumentation became readily available.
I was a tool room foreman for a company in the hydraulics industry and I am well aware of this. Half an inch plus or minus a tenth (of a thou) or plus or minus the snoot of your bonnet. There is no such thing as "exactly the same size".
As for general aviation navigational tolerances. If you can see the target at the calculated time it's close enough innit? Being retired from all that is a piece of cake, though I still have my own machines to play with, plus or minus not a lot as required.
19th Apr 2015, 02:04
#40 (permalink)

Join Date: Mar 2001
Location: Toronto, Ont, Canada
Posts: 223
I also received some good lessons in the use of precise terms such as the difference between 2 1/2 and 2.5. I once went to the Toolroom foreman, a God in the motor industry in the 60's, and asked for a strip of steel 0.5 in. wide.

He asked if I really wanted it 0.5 in. wide and I said yes, half an inch. "But you've written 0.5 in.", he said rather scornfully. "Yes, I said, Half an inch" .

Eventually, he patiently explained to me the error of my ways, a lesson I have never forgotten.
My feeble math challenged brain doesn't "get it"

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