CHICAGO, Aug 14 (Reuters) - Airlines are getting grounded jets back into the air. The bills left by a years-long engine crisis are proving much harder to bring down.

Air New Zealand ( ANZFF ) offers a glimpse of how those costs can linger. Engine problems at one point left as much as 20% of its fleet unavailable, forcing it to lease extra aircraft and engines to protect its schedule.

Availability has since improved sharply, but Chief Executive Nikhil Ravishankar told Reuters it could take 12 to 18 months to shed the extra leases and related costs, only partly offset by supplier compensation.

The crisis was fueled by durability problems affecting some newer engines and a Pratt & Whitney powder-metal problem that forced accelerated inspections and removals, compounded by shortages of labor, parts and repair capacity. Airlines leased replacement engines and aircraft to keep flying, leaving them with higher overhaul, parts and lease costs even as groundings ease.

Delayed deliveries from Boeing ( BA ) and Airbus are adding to the pressure by keeping older jets in service longer and pushing some airlines into engine work they had expected to avoid.

A Reuters analysis of U.S. Transportation Department data found that across six large U.S. airline operations, reported spending on engine labor, aircraft-engine repairs and engine materials rose about 68% between 2019 and 2025, while hours flown increased about 10%.

The same pattern appeared in the latest available Transportation Department data, with reported spending in those categories up 17% in the first quarter from a year earlier while hours flown increased less than 2%.

Maintenance spending can vary with engine age, accumulated cycles, fleet-management decisions and the timing of shop visits, during which engines are removed from aircraft for inspection or repair. The Transportation Department data do not identify what caused the increase.

GE Aerospace, Safran and RTX did not respond to requests for comment. Rolls-Royce declined to comment.

RTX's Pratt & Whitney illustrates the disconnect. Groundings tied to its geared-turbofan engines, which power many newer single-aisle jets, fell 25% in the first half. But in the second quarter, the mix of heavier PW1100G repair work was 14 percentage points higher than a year earlier.

Latest regulatory filings from United Airlines and American Airlines ( AAL ) also show higher first-half maintenance expenses driven partly by engine-overhaul work.

Parts and other materials account for about 60% of the direct cost of a typical single-aisle engine overhaul, according to consultancy Oliver Wyman. Newer engines deliver fuel-efficiency gains, but overhaul bills are higher because they have fewer developed repair options and fewer used parts available, partner Sam Sargent said.

George Dimitroff, head of valuations at Ascend by Cirium, said overhaul and mandatory parts-replacement costs for newer LEAP and GTF engines, which power most Airbus A320neo- and Boeing 737 MAX-family jets, have risen about twice as much since 2019 as for older CFM56 and V2500 engines.

Not all the maintenance growth on newer engines is tied to technical problems; maturing fleets also require more scheduled work.

LEASES OUTLAST GROUNDINGS

Long shop visits can force airlines to lease replacement engines.

JetBlue Airways ( JBLU ) has said some Pratt engines can take 200 to 300 days to complete a shop visit. The airline has also increased the number of engines it leases.

Short-term leases for some newer LEAP and PW1100G engines have exceeded $6,500 a day in completed transactions over the past year, compared with around $5,000 a day in 2022-23, according to aviation consultancy IBA.

Austin Willis, CEO of engine lessor Willis Lease Finance ( WLFC ), told Reuters airlines needing replacement engines have often sought leases lasting around three years, while shorter contracts are frequently extended. That can leave airlines paying even after their own engines return.

OLD JETS, NEW BILLS

A full overhaul of a CFM56-5B engine used on older Airbus A320-family jets can easily cost more than $10 million, Willis said.

Oliver Wyman and the International Air Transport Association estimated that delayed aircraft replacements could have added about $3.1 billion to global airline maintenance costs in 2025.

Delayed aircraft retirements are also tightening the supply of cheaper used engine parts. Industry officials and experts say fewer engines are being dismantled for parts, pushing up used-parts prices and leaving airlines more dependent on new parts from manufacturers.

PRICING TENSIONS

The squeeze has sharpened tensions between airlines and engine makers over repair costs and pricing.

Asked at the International Air Transport Association annual meeting in June whether engine makers had too much pricing power and were using shortages to charge airlines more, United Chief Executive Scott Kirby replied: "Yes and yes."

Industry executives say developing new technology costs billions of dollars and manufacturers often discount engines heavily upfront, expecting to recover that investment over years through parts and maintenance.

Safran CEO Olivier Andriès last month said increases in repair and spare-parts prices should be moderate and reflect higher supplier costs rather than an "abusive posture."

GE Aerospace CEO Larry Culp said last month that LEAP-related aircraft groundings had fallen to near zero. GE is also rolling out upgraded parts designed to let the engines stay on aircraft longer before needing major maintenance.

RTX, which said it is improving durability and expanding repair capacity, reported Pratt's PW1100G repair output rose 43% in the second quarter from a year earlier and turnaround times fell 23%.

For airlines, the financial relief can take longer.

"The fact that AOGs (aircraft on ground) have come down, that doesn't change the maintenance cost," Willis said.