# Talk:Zero-fuel weight

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## Unclear wording

"For example, if an airplane is flying at a weight of 5,000 lb and the weight of fuel on board is 500 lb, the Zero Fuel Weight is 4,500 lb. Some time later, after 100 lb of fuel has been consumed by the engines, the total weight of the airplane is 4,900 lb and the weight of fuel is 400 lb. The Zero Fuel Weight is still 4,500 lb"

The first part of the above statement makes perfect sense. However it appears some funny math is going on in the second statement.

IF we have 5000 lbs and we substract 500 lbs of fuel the total weight of the plane is 4500. From here the equation does not balance above. Because if we have burned 100lbs of fuel of the 500lbs then our fuel weighs 400lbs. Now according to the above way of initially calculating things ---the total weight would be 4600. Unless 10-4= 9? which is of course wrong.

Now it does make perfect sense if we ignore all the above and state that the total weight is 5000 lbs and when we have flown a bit and have consumed 100 lbs of fuel then the total weight of the plane is then 4900 but not as written in the gobbly gook lingo in the second sentence

The weight of the plane with no fuel is 4500. Add 500 lbs of fuel and the total weight is 5000 lbs.

Perhaps you mean that the total weight of the plane is 5000 lbs and if we take away 100 lbs fuel weight then the weight of the plane minus the fuel consumed is 4900 and the total weigh of the fuel now is 400 lbs because we have burned up 100 lbs from when the total fuel weight was 500 lbs???.

this article needs to be cleaned up and clearly definited and re-written without technical gobbleygook lingo so that a child can understand it with no advance math such as graphs et al. —Preceding unsigned comment added by 24.197.124.167 (talk) 18:37, 21 September 2008 (UTC)

The statement you have quoted is correct. A couple of things you have written are very puzzling.
For example, you have written If we have 5000 lbs and we substract 500 lbs of fuel the total weight of the plane is 4500. The example states that the aircraft is carrying 500 lb of fuel. If, as you say, we substract 500 lbs of fuel, the aircraft has now exhausted all its fuel and the engine stops. Is that what you have in mind?
As another example, you have written Because if we have burned 100lbs of fuel of the 500lbs then our fuel weighs 400lbs. Now according to the above way of initially calculating things ---the total weight would be 4600. The aircraft begins at 5000 lb. If we burn 100 lb of fuel the total weight of the aircraft would be 4900 lb, not 4600 lb.
I suspect you have not fully grasped the meaning of Zero Fuel Weight. I suggest you read the article carefully. Cheers. Dolphin51 (talk) 23:28, 22 September 2008 (UTC)

I thought zero-fuel weight only excluded the weight of *usable* fuel. The article says it excludes the weight of all fuel on board. The FAA's "Airplane Flying Handbook" confirms this. "Zero fuel weight is simply the maximum allowable weight of the airplane and payload assuming there is no usable fuel on board ... all weight in excess of that figure [maximum zero-fuel weight] must consist of usable fuel." JoelKatz —Preceding undated comment added 10:24, 17 June 2009 (UTC).

This article is about zero-fuel weight in general terms. The article is not tailored to any particular airplane type or airplane manufacturer. In general terms, zero-fuel weight is the weight of the airplane at any moment in time, minus the weight of the fuel at that moment in time. For a more precise definition for a particular type and model of airplane the pilot should consult the aircraft flight manual or Pilots Operating Handbook. Different manufacturers are likely to define it differently. The weight of unusable fuel is a very small fraction of the airplane weight.
There is a similar difference in the definition of empty weight. Older airplanes use a definition of empty weight that involves zero engine oil; modern airplanes have their empty weight based on full engine oil. The weight of engine oil is a very small fraction of the airplane weight.
You have written Zero fuel weight is simply the maximum allowable weight ... ...
You are confusing zero-fuel weight with maximum zero-fuel weight (MZFW). MZFW is a fixed weight; it is published in the aircraft flight manual and never changes. Zero-fuel weight (ZFW) changes as payload is added to, and removed from, the airplane. ZFW is different on every flight. ZFW is not permitted to exceed MZFW. Dolphin51 (talk) 11:57, 17 June 2009 (UTC)
"In general terms, zero-fuel weight is the weight of the airplane at any moment in time, minus the weight of the fuel at that moment in time." No, it's not. It's the weight of the airplane at any moment in time minus the weight of the usable fuel at that moment in time.
"Different manufacturers are likely to define it differently." Nevertheless, the FAA defines MZFW as the weight above which all weight must be usable fuel. This means it cannot include unusable fuel. If you can cite *anyone* who defines it differently, please do. The article contains no citations, so I picked the most authoritative one I could find (the FAA). But I have many others that are all consistent -- empty fuel weight and maximum empty fuel weight simply do not ever combine the weight of usable and unusuable fuel.
"This article is about zero-fuel weight in general terms." Simplifying is fine. Being incorrect is not. ZFW is not "the total weight of the airplane and all its contents, minus the total weight of the fuel on board" as the article claims. It is minus the total weight of the usable fuel onboard.
I have no idea why you think I'm confusing ZFW with MZFW. Maybe it's just because I put the '...' in an odd place. I was trying to show the FAA does not combine usable and unusuable fuel weight for either measure. The first part is about ZFW, the second about MZFW. Note that both qualify "fuel" with "usable". —Preceding unsigned comment added by JoelKatz (talkcontribs) 14:51, 17 June 2009 (UTC)