'Both the A380 and B787 are rated as producing a quarter of the noise level of the B747-400.'
What is being talked about is the Heathrow noise quota count.
747-400 QC2
A380/787 QC0.5
So you can land four A380/787s for every 747-400 allowed under the night quota total.
Well, not quite as simple as that as there is a total movement limit as well as a total quota limit.
