I once had the opposite problem (so did everyone else on the task). We were on a multi-aircraft Cold War exercise. The squadron task was to fly underslung loads from one field location to another, by night. The loads were a number of Rapier ground to air missiles in transit cases. These loads were classified as "cleared loads" which meant that JATE had carried out flight trials and the weights, rigging details, maximum flight speeds etc. were laid down in the military helicopter underslung loads manual so we planned our (reduced) fuel loads accordingly.
That night there was a strong wind blowing, against us on the load carrying leg. About 45 to 50 kts at our flight altitude of about 1,000 feet agl. Not normally an issue, except one had to bear in mind that the Puma was normally short of fuel before takeoff even with full tanks!
This shouldn't have been a problem; it was all planned for. However, when we picked up the loads it was noted from the collective pitch used to hover with them hanging underneath that they were much lighter than notified. Because the loads were so light they wouldn't fly above about 55 kts before becoming totally unstable and swinging very violently, so 55 kts IAS it had to be; not 90 as planned for! Cue a long night stream of half a dozen Pumas trogging along into wind but making very little headway. I watched two of the formation chicken out due to fuel shortage on sighting someone's NATO "T" down below. Being braver and probably more stupid than I am now, in view of the perceived value and military sensitivity of the load, I carried on and got the load to intended destination with both fuel low level lights on.
A later investigation discovered that the Army hadn't actually put any missiles in the boxes. They should have ballasted the boxes with sandbags to make up the equivalent weight but didn't bother. I nearly bust my backside for a few empty boxes!
