Airport passenger numbers for the current year are expected to reach 94,000.
The figure is up on last year but less than the Council had predicted.
A report on Airport Fees & Charges for today's Full Council meeting states that an assumption was made last year that passenger numbers would continue to rise and "based on the most recent trends anticipated to reach 98,000".
It added: "Unfortunately passenger numbers have been less than anticipated during 2018/19 and likely to reach only 94,000."
Report author Craig Dryden, Senior Manager for Infrastructure & Planning, "optimistically" estimated that passenger numbers for 2019/20 will reach around 97,000, "reflecting a modest growth (3%) in the market based on projected trends since 2013/14".
The Council’s Lead Member for Place Steve Sims said: “It has proved very difficult to make accurate projections for airport numbers but they have to be made to present a budget. Passenger numbers have improved since the low point in 2013/14, the first financial year after BIH flew away.
"I think the 3% estimated growth for next year seems reasonable given the weather last summer and the probable weakness of the pound post-Brexit. But I hope it woefully undershoots the actual figure. With numbers much below 100,000, the future viability of the airport is at risk. There has to be a balance between compensating low numbers with increased charges, because this further undermines any operators whose margins will already be low."
He added: “Numbers are beyond the airport's control and we should all remember the multiplier effects of us having an airport far outweigh a simple look at the books. Sadly on Scilly we are not in a position to subsidise the airport to the extent that Cornwall Council subsidies Newquay, so let’s hope for good numbers next year, which will be great for the airport and the islands in general.”
Airport passenger figures peaked in 2002 at 148,346 (with an additional 44,558 flying to Tresco) and hit a low of 86,603 in 2013/14.
Passenger numbers over recent years:
2013/14 - 86,603
2014/15 - 92,997
2015/16 - 95,441
2016/17 - 96,007
2017/18 - 92,975
2018/19 - 94,000 (forecast)