Defence sources are confident it will be possible to fly sufficient RAAF "classic" Hornet jet fighters long enough to avoid an air defence capability gap at the end the decade.

The RAAF has 24 brand new F/A-18F Super Hornets and 71 of the F/A-18A-B "classic" Hornets dating back to the 1980s.

The 71 "Top Gun" era fighters were due to be retired in 2018 but, as a result of delays in the Joint Strike Fighter program, some are now expected to be kept in service until 2021.

All of the "classic" or "legacy" Hornets are undergoing a major capability upgrade as part of the AIR 5376 program.

This includes the installation of an electronic warfare modification program which includes a new radar warning receiver, an extra countermeasures dispenser, electronic countermeasures systems and more computer memory. Full delivery of the work is scheduled to occur in 2013.

The "classic" Hornets are also well on the way to getting an even more lethal sting following the successful test firing of a Joint Air to Surface Stand-off missile at the Woomera range in July.

Ordered by the Howard government in 2006 ahead of the retirement of the F-111, the JASSM has a range of more than 200km and can shatter hardened concrete bunkers.

Defence insiders say there will be no need to keep all of the "classic" Hornets flying until the very last of the expected 100 JSFs arrives.

As each JSF squadron comes on line an equivalent number of F/A 18A-Bs will be stood down.

The plan, as spelt out in the latest update of the Defence Capability Plan issued earlier this week, is for Australia to have its first battle-ready JSFs in 2018.

The first and second squadrons are to be based at Williamtown.

The third squadron will be based in Tindal with a fourth at Amberley.

It is expected that three squadrons will have achieved Initial Operating Capability by 2021.

Defence's 2010-2011 annual report noted the Defence Materiel Organisation had been very successful in keeping the "classic" Hornets in the air - despite emerging issues such as corrosion and component failure.

"Recovery plans were successfully developed and implemented to manage these issues," it states.

"On average, since February 2011, DMO has met the Air Force's requirements for available and operations F/A-18 aircraft."