# How do you avoid power of 10 or order of magnitude errors when using an E6B calculator?

I am currently working through Gleim's instructions for using their E6B calculator.

It says: "The numbers on the inner and outer scales represent multiple sof 10 of the values shown." And "EXAMPLE: The number "20" on either scale me represent 0.2, 2.0, 20, 200, or 2,000".

I've been working through the examples, and a recurring problem is getting the right value to multiply by 10, but not knowing what power of 10 to multiply it by. (To put it in math terms: right mantissa, wrong exponent).

What is the process or rules to make sure you get the right answer every time? Obviously, getting a flight calculation wrong by an order of magnitude is not a problem you want to have!

It's not a problem you'll ever have in practice, as it will always be obvious to you whether that "2" on your wizz wheel is 0.2, 2, 20 or 200. It is not a problem you'll have with time/endurance etc issues as the inner scale of time is unmistakable.

But lets take some practical examples:

Conversion Nautical to statute miles

• Set 90 next to the "NAUT" marking
• Read the value next to the "STAT" marking. it is somewhere about 10.5.
• You know that nautical miles and statute miles are fairly similar
• You therefore know this cannot be 10 Statute miles, nor 0.1 so must be around 100

Calculate speed from distance and time

• Distance is 26NM and time is 13 minutes
• Set 13 on the inner scale opposite 26 on the outer scale
• The rate arrow points at the speed. It says 12.
• I'm flying a standard single engine prop. Can it fly at 12 Knots? Can it fly at 1200 Knots?
• The answer can only possibly be 120Knots

Fuel burn rate

• You took 32 gallons of fuel and flew 4 hours 20 minutes
• Set 4:20 on the inner time scale against 32 on the outer scale
• Rate arrow points at 74.
• Did I burn 74 Gallons per hour? My Cessna only holds 40! Did I burn 0.74? That seems awfully low - with that my endurance would be measured in days when I know its roughly 4 hours.
• The answer can clearly only ever be 7.2 Gals/hour burn rate

The thing with being out by a factor of 10, is that it is always obvious you're out by a factor of 10. 2x, 3x even 4x you might get confused. But I don't think you ever will with 10x

The even shorter answer is that once you've gained your PPL you'll only ever pull out your E6B to answer these such questions, and any other time you'll use modern, electronic tools to answer these questions. Slide rules; gone from the classroom in the 1970's but still used to teach people to fly.

• In other words: "Common sense". +1 Dec 1 '20 at 16:29
• @JörgWMittag yes, but this was the nicer way of saying that!
– Jamiec
Dec 1 '20 at 18:08
• Plus mentally doing a round-off approximation gives you the rough answer including the order of magnitude. The E6B or slide rule just gives you a few digits more accuracy. For your examples, NM is close to SM so 90NM is close to 90SM - or, in this case, 103.5SM. 25 miles in 1/4 hour is about 25*4 or 100 MPH. 32 gallons in a little over 4 hours is a little less than 32/4 or 8 GPH. Dec 1 '20 at 21:06