# What is the difference between dry operating weight and dry operating mass?

What is the difference between dry operating mass and dry operating weight in the context of rotorcraft?

What is the formula for converting mass to weight?

• You should provide the context, as in aviation weight is often used where mass would be more accurate. w=m*9.81 at sea level and at the equator. Mass unit is the kilogram, weight unit is the newton. – mins May 25 '16 at 17:37
• First you weigh the object on the moon, then you divide that number by 0.167 and that is the mass. Getting the aircraft to the moon is the tough part. – Tyler Durden May 25 '16 at 18:13
• On Earth, there is, for practical purposes, no difference. – Sean Oct 11 '19 at 5:54

Weight is a force.

Mass is a property of matter. Acceleration makes mass produce a force. Specifically, a mass in the gravitational field of Earth effects a weight force on its support. The relation between both is the gravitational acceleration g, which is standardized to 9.80665 m/s² (approximately 32.174 ft/s²), as in Weight = Mass · g.

The problem occurs due to lazy terminology. In aviation both normally mean the same thing: The reading you would get if you put the empty aircraft onto a large weighing device (scales, or whatever). This is normally given in kilograms (kg) or pounds (lb).

This trouble arises because both of these quantities are actually masses, the weights would be in Newtons (N) or pounds-force (lbf). To transform the mass into a weight you multiply by the local gravity, g, which is usually assumed to be 9.81m/s2.

A similar problem exists when at home: If you weigh yourself on standard bathroom scales you will normally see a value in kilograms (i.e: A mass), but the scales actually measure a force. This means if you use the same scales on another planet you will get a different value in kilograms, which is nonsensical. The same goes for aviation: Weight and mass are being used interchangeably, which is incorrect.

• edit - And to specifically answer your question: Dry weight and dry mass mean the same thing in this context.

Weight = Mass x Gravity.

In typical aviation terms, in an operational sense, you don't need to worry about mass (unless you're talking about a spacecraft...)

• If I need to accelerate with marginal thrust and a limited runway, I very well worry about mass. – Peter Kämpf May 26 '16 at 7:38
• @PeterKämpf well you may worry about mass, I will worry about weight ;) – Ben May 27 '16 at 10:10
• OK, Ben, I will type more slowly. Maybe that helps. When you want to take off, you need to gain speed. Now the acceleration that acts on the aircraft's mass is not only gravity, but the acceleration gained by thrust. If thrust is low, so is acceleration. If the available runway is limited, this might very well make a takeoff impossible. Sure, a lower weight will also require less speed to fly, but during a takeoff run mass is doubly harmful. – Peter Kämpf May 27 '16 at 19:19
• It looks like I've offended you - sorry, that wasn't my intention. The point I was trying to make, which perhaps sp1 has better articulated, is that in typical operational circumstances, 'weight' and 'mass' are used interchangeably (even though they do have distinct meanings) when they are just trying to say weight. I'm not saying the mass of the aircraft doesn't affect anything. – Ben May 28 '16 at 12:25