Replacing ageing military equipment is often fearsomely expensive. As much has been re-emphasised by the visit to this country of a Royal Australian Air Force Boeing C-17 Globemaster. The aircraft, seen as a possible successor to the air force's five Hercules C-130s and two Boeing 757s, was test-driven by members of Parliament's foreign affairs and defence committee this week. Any enthusiasm would have been tempered by the $250 million price attached to the giant aircraft.

That tag seems to have persuaded the New Zealand First defence spokesman, Ron Mark, that this country can do without them. If it wanted to take troops long distances, it should be able to call on "the club", the Prime Minister's description of its American, Canadian and Australian allies, he said. Wielding the begging bowl would not, however, be wise. There will be times when this country's interests and priorities are different from those of its allies. Its air force should be able to carry troops on peacekeeping operations or deliver humanitarian aid when and where they are needed.

Equally, not having that capacity risks making New Zealand the butt of jokes. Already, the Hercules, three of which are 50 years old, have ventured into that territory. In 2003, for example, the dispatch of military equipment to the Solomons had to be delayed because all four of the Hercules then in New Zealand were out of action. Subsequently, some $170 million has been spent upgrading the aircraft. But if there have been fewer embarrassing breakdowns, the Hercules are an ongoing challenge for the air force's engineers.

The Boeing 757s have also had their problems. In 2013, one of them, with Foreign Affairs Minister Murray McCully on board, had to make an emergency landing at a fog-enclosed Antarctic airfield. Like the Hercules, the 757s do not have the range to be able to turn back to Christchurch if they pass a point of no return. The Transport Accident Investigation Commission said the incident raised questions about the suitability of the 757s for Antarctic operations.

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That indicates both the Hercules and the 757s should be replaced by an aircraft more suited to the needs of New Zealand in the interests of a balanced Defence Force which has the flexibility required for its sphere of operations. A decision must be made soon. The upgrading of the Hercules was intended to give them 15 more years of life, a period that expires in 2018. The choice is between an upgraded version of the Hercules, at a cost of $90 million to $100 million each, and the Globemaster which, at more than twice the cost, can fly faster and further than the Hercules and land on shorter runways.

There are strong arguments for both aircraft. Over many years, the Hercules have proven themselves the durable workhorses of many air forces. In disaster zones, they may be more flexible. Cost is obviously also a strong point in their favour. The versatile Airbus A400M is another possibility. Either way, the Government must not abandon a heavy airlift capacity.

Not if it wants a credible Defence Force.