Hello all,

Rather a simple question and maybe a bit off-topic but I'm wondering why the climb gradient equation is (RoC in ft per min)/GS?

These units aren't the same so how can it possibly be useful for obstacle clearance? I got a question asking for climb gradient - converted 1000fpm into nautical miles (=0.1645...nm) then divided GS by 60 to get the nautical miles travelled in one minute. I got close to the answer but incorrect and obviously I've used the wrong formula.

Just wondering from a philosophical standpoint why we use RoC/GS.

The rate of climb formula is an approximation.

By a lucky accident of arithmetic, if you leave the Ground speed in knots, the answer becomes a percentage.

Assuming Rate of climb is 1000ft/minute and ground speed is 100 knots.
Using 6080 ft/nautical mile gradient = 9.9%
Using 6000 ft/nautical mile gradient = 10%

The pilot has the rate of climb as a continuous readout on the VSI
The pilot has the ground speed as a continuous readout on the GPS.
Climb gradient = VSI reading in feet per minute ÷ GPS ground speed in knots.
It should be a simple matter to check your climb gradient if ever you need to for the sake of an exam question.
I very much doubt if you will ever need to do it in the real world!!

I'm wondering why the climb gradient equation is (RoC in ft per min)/GS? These units aren't the same so how can it possibly be useful for obstacle clearance?

Hazza,

I presume you have a STEM background by virtue of the fact that you picked up the inconsistent unit matter. I further presume that your calculations were mathematically sound and correct. In that case, your answer is the correct answer.

However, with piloting stuff we don't have time to attend to all the details, all the time, so we take approximating shortcuts wherever that might be reasonable and useful. This calculation is just one such instance.

Bob's reply gives the story according to pilot wisdom. The answer is not mathematically correct but an error of around 1% is fine and quite acceptable for pilot stuff. His reply tells the sequence story of what is going on.

I very much doubt if you will ever need to do it in the real world!!

Just as an aside, while Bob's observation is quite pertinent in the GA world, should you end up on heavy iron, you will find that, for the considerably more complex takeoff calculations involved in Part 25 matters, we actually do routinely know what target minimum climb rate we are looking for, especially in the OEI case. That is the only practical way to keep an eye on what the aircraft might be doing, climbwise and is very near and dear to the heavy iron pilot when there are lots of nasty bumpy things not too far under the aircraft's keel.

Interesting and definitely convenient, thanks for the info guys!