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

Thread Starter

Join Date: Sep 2014

Location: Earth

Posts: 27

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

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

I understand the answer is 4.08NM

But I want to know how one would calculate the answer?

I understand the answer is 4.08NM

But I want to know how one would calculate the answer?

Join Date: Aug 2011

Location: UK

Age: 49

Posts: 251

I'm hoping he meant an easy way to calculate it in his head... There are 60 minutes in an hour. So 98kts is just a little over 1.6kts per minute x 2 then add .5 which equals 4. Close enough for doing the calculations in your head.

Join Date: Jan 2011

Location: England

Posts: 643

The profile for Spencer indicates that he is currently working towards his PPL so we should not be too surprised that he is asking this type of question.

1 knot is 1 nautical mile per hour.

This means that 98 knots is 98 nautical miles per hour.

There are 60 minutes in an hour so if we divide 98 by 60 we will convert 98 nautical miles per hour into 1.633 nautical miles per minute.

If we then multiply this by 2.5 minutes we will get the number of nautical miles flown in 2.5 minutes. 2.5 X 1.633 is 4.0825

1 knot is 1 nautical mile per hour.

This means that 98 knots is 98 nautical miles per hour.

There are 60 minutes in an hour so if we divide 98 by 60 we will convert 98 nautical miles per hour into 1.633 nautical miles per minute.

If we then multiply this by 2.5 minutes we will get the number of nautical miles flown in 2.5 minutes. 2.5 X 1.633 is 4.0825

Join Date: Jan 2002

Location: Aberdeen

Posts: 1,233

It should be fairly obvious from the replies above, for a no wind day - but wind will completely change the situation - hence the infamous 'whizz wheel' where you can graphically add the wind vector and then derive groundspeed.

Join Date: Aug 2005

Location: Glens o' Angus by way of LA

Age: 56

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If I have this wrong I will gladly be corrected.

Join Date: Mar 2014

Location: The World

Posts: 1,229

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

I understand the answer is 4.08NM

But I want to know how one would calculate the answer?

I understand the answer is 4.08NM

But I want to know how one would calculate the answer?

On the ground, real calculation -> 98 knots equal 98 nautical miles per 1 hour equal 60 minutes, so the 2.5/60 fraction of 98 nautical miles = (2.5/60)*98 = 4.083 nauticals

In the air, head calculation -> 98 knots is almost 90 knots +10% and 90 knots is 1.5 nauticals per minute, 1.5 times 2.5 is 3.75 plus 10% roughly equals 4.1 nauticals - good enough

Join Date: Jan 2008

Location: Canada

Posts: 602

The most memorable math class I ever had in school saw a surprise test. It was simple math, but big, awkward numbers, and lots of them. The teacher told us that we would not have time to calculate them all (and calculators - to be honest, had not been invented yet). He told us that nearly all the math we would ever do in life would be used to make a decision. The result would direct us to a "yes" or a "no". So, if we were pressed for time, a close guess would probably result in the correct decision. The purpose of the test was for us to guess at the answer. If our guess was within 10% of the correct answer, we would get the mark.

This logic is very prevalent in piloting. You'll need to make many decisions, most usually, do I have enough fuel to get there? The good guess, with a bit of conservatism, will be adequate.

I do not imagine a situation where based on the groundspeed of a GA aircraft, there is a need to know the distance covered to within 1/100 of a mile. For my experience the nearest mile, or 5% for greater distances has met the need every time. That said, during training and exams, the questions might force greater precision, as you choose the best of two possible multiple choice answers. Know the theory, but know when to apply it too!

This logic is very prevalent in piloting. You'll need to make many decisions, most usually, do I have enough fuel to get there? The good guess, with a bit of conservatism, will be adequate.

I do not imagine a situation where based on the groundspeed of a GA aircraft, there is a need to know the distance covered to within 1/100 of a mile. For my experience the nearest mile, or 5% for greater distances has met the need every time. That said, during training and exams, the questions might force greater precision, as you choose the best of two possible multiple choice answers. Know the theory, but know when to apply it too!

Join Date: Sep 2006

Location: 23, Railway Cuttings, East Cheam

Age: 64

Posts: 3,121

I'm not saying the OP should ball park it for his PPL studies but once in the real world just divide it up into multiples of 6.

90 kts=1.5 nm minute

96kts=1.6 nm minute

102kts= 1.7nm minute etc.

98 kts is nearest to 96 so just multiply 1.6 by 2.5 which equals 4.0. If you're feeling particularly anal bung a bit on for the wife and kids.

Piperboy: you don't have it wrong, I use the same method. It's just the sine of the wind angle. 60 degrees off would actually be 0.866 but it's near enough to one for me. I use the same method for xwind component on takeoff/landing, which is what your drift angle is anyway.

Come on, wouldn't be Prune without people jumping in at the deep end!

90 kts=1.5 nm minute

96kts=1.6 nm minute

102kts= 1.7nm minute etc.

98 kts is nearest to 96 so just multiply 1.6 by 2.5 which equals 4.0. If you're feeling particularly anal bung a bit on for the wife and kids.

Piperboy: you don't have it wrong, I use the same method. It's just the sine of the wind angle. 60 degrees off would actually be 0.866 but it's near enough to one for me. I use the same method for xwind component on takeoff/landing, which is what your drift angle is anyway.

Some the answers / remarks given here fall in to the category of RTFQ, were does the wind come in to the calculation ?

Join Date: Mar 2014

Location: The World

Posts: 1,229

Guys, keep it stupid simple ... lets take the easy road with ground meaning no wind, perpendicular to gravitational forces -> if it would be groundspeed along gravitational force the typical GA plane won't make 2.5 minutes ...

Join Date: Aug 2005

Location: Glens o' Angus by way of LA

Age: 56

Posts: 1,974

The most memorable math class I ever had in school saw a surprise test. It was simple math, but big, awkward numbers, and lots of them. The teacher told us that we would not have time to calculate them all (and calculators - to be honest, had not been invented yet). He told us that nearly all the math we would ever do in life would be used to make a decision. The result would direct us to a "yes" or a "no". So, if we were pressed for time, a close guess would probably result in the correct decision. The purpose of the test was for us to guess at the answer. If our guess was within 10% of the correct answer, we would get the mark

Bottom line, I thought to myself, is she cant start out with a ballpark number in her head and then work the actual figures that will verify her "guesstimate" then she is going to bankrupt me sooner or later and out the door she went.

Always start out with a rough mental calculation before working out the exact number.

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Join Date: Aug 2006

Location: Ontario, Canada

Age: 59

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The discussion about precision is interesting. Though by no means an expert in math, I have some familiarity with precision and rounding numbers. The OP presented the time as "2 1/2 minutes". This would presumably equate to 2.5 minutes. Therefore the actual time value could be 2.25 to 2.74 minutes, if I have it right. Thus, working out the distance to 0.01 miles would be not entirely appropriate, if the time is being expressed to only 0.1.

But, that's semantics, for entertainment. Consider why you need the answer, and that may point the required precision. In this case, to pass a groundschool exam, you may as well get the number right on - just know that in many cases in the real world, that's not needed...

But, that's semantics, for entertainment. Consider why you need the answer, and that may point the required precision. In this case, to pass a groundschool exam, you may as well get the number right on - just know that in many cases in the real world, that's not needed...