OTTAWA -- The Harper government has signed off on a proposal to buy an additional C-17 Globemaster heavy-lift transport plane, bringing to five the number of the hulking airborne workhorses in the air force fleet.

Multiple defence sources say the decision to acquire the aircraft was made recently, but has not yet been announced as officials are still hammering out the contract details.

The Canadian air force has been pushing for the deal for over two years, telling senior officials that there is "a strong operational and business case" for a additional aircraft given how much use the fleet has gotten since it was introduced in 2007.

Earlier this year, The Canadian Press reported that National Defence believed it could afford the estimated US $169-million price tag because it had not spent all of the funds made available for the initial purchase of four C-17s.

Documents obtained under the Access to Information Act show the department had allocated $1.8 billion to complete the first purchase, but only $1.4 billion was spent.

There is urgency because Boeing stopped production on the workhorse aircraft last summer, and published reports in the U.S. indicate as many as 10 of the gigantic planes were up for sale.

The U.S. government announced in 2006 that it would stop buying the aircraft. The company has relied on foreign sales to keep the plant that produces the Globemaster operating in Long Beach, Calif.

Boeing refused comment on Wednesday.

Australia recently committed to buying two of the remaining aircraft, but diplomatic sources said this week Tony Abbott's government is expected to go even further next year and buy additional C-17s.

In its briefings to government, the military described the chance to procure another one as a "unique, time-sensitive opportunity for DND."

The air force argued that adding another C-17 would expand its capability in disaster and humanitarian missions, while also easing the burden on the existing fleet.

"Canada's experience in Afghanistan and other theatres of operation has shown that fifth (C-17) aircraft would prove a highly beneficial asset to the Canadian Forces," said the briefing document, dated Feb. 14, 2012.

National Defence and Public Works and Government Services Canada have struggled to deliver a whole host of hardware, including ships, army trucks, helicopters and fixed-wing search-and-rescue planes.

Buying an additional C-17 would be comparatively easy, since it would likely involve a sole-source deal with the manufacturer, arranged through the U.S. government.

The Globemaster is an expensive aircraft to operate, according to U.S. Air Force comptroller and defence industry data. It is estimated to cost US $23,279 per flying hour, according to 2012 estimates.

The C-130J Hercules, the other military transport recently purchased by the air force, rings in at US $13,644 per flying hour.

The possible purchase of another C-17 was not listed in the government's defence acquisition guide, released last June.

We already have Herks.

We're into our second generation of Hercs.

Like the ones in France?