Few aircraft have come closer to cancellation and still entered service. Delays and design changes drove the expected cost of a Marine MV-22 from about US$24 million per aircraft in 1986 to an estimated US$84 million by 2009, according to Air & Space Forces Magazine, and Defence Secretary Dick Cheney tried to kill the programme in 1989. Congress kept it alive. In 1989 the Pentagon was still planning for 663 aircraft; by 1997 the number had settled at 458, a figure the Congressional Research Service tracked through the following decade.

The worst blow came in 2000. In April a V-22 descended too quickly with too little forward speed, entered vortex ring state and crashed at Marana, Arizona, killing 19 Marines. After another fatal crash in December the fleet was grounded and the programme looked finished. On the advice of a blue-ribbon panel the Pentagon gave it one more chance. Bell and Boeing reworked the flight-control software and hydraulics, and 18 months of increasingly demanding test flying showed that the tiltrotor was in fact less susceptible to vortex ring state than conventional helicopters and, because its nacelles can tilt forward, recovered faster. Aircraft now carry visual and aural warnings of the condition, and pilots are trained to recognise it. Full-rate production was approved on 28 September 2005, and the Marines deployed the type to Iraq two years later.

The first operational MV-22s went to Al Asad in Iraq’s Anbar province in 2007. They flew troops, cargo and casualties between forward bases, and senior commanders quickly realised that speed and altitude kept them above much of the small-arms fire that threatened helicopters. One aircraft carried Senator Barack Obama during his visit to Iraq in 2008. The deployment was not smooth: the Congressional Research Service recorded mission-capable rates of about 68, 57 and 61 percent for the three squadrons, short of the 82 percent requirement, and squadrons cannibalised parts because components wore out far faster than predicted. In Afghanistan from late 2009 the Osprey flew assault, resupply and casualty evacuation, its speed getting more wounded to surgery inside the “golden hour”.

The most dramatic Air Force mission came on 21 December 2013. Three CV-22s were sent to Bor in South Sudan to evacuate US citizens from a civil war, their crews told to expect a permissive landing zone. As they approached they were hit 119 times by AK-47 and .50-calibre fire; four Navy SEALs in the lead aircraft were wounded. All three Ospreys, leaking fuel and hydraulic fluid, diverted to Entebbe in Uganda. The incident led Air Force Special Operations Command to request removable armour: NAVAIR fielded a kit of 66 plates, adding about 800 lb, within 179 days.

Each nacelle carries a Rolls-Royce AE 1107C turboshaft, designated T406 in US service and marketed as the Liberty, rated at 6,150 shp. The two engines are linked by a driveshaft running through the wing, so if one fails the other can turn both proprotors. That protects against asymmetric thrust, but the V-22 is generally not able to hover on one engine, and its light, stiff rotors have poor autorotation characteristics; in a total power loss crews are trained to convert to aeroplane mode and glide. The wing itself is short and thick to carry the nacelle loads, and the airframe makes heavy use of composites.

The arrangement has been refined continually. Rolls-Royce added a new turbine and software to raise power in hot and high conditions, and Bell’s recent Nacelle Improvement programme reworked wiring and structure inside the nacelles to cut maintenance hours, a long-standing weak point. Bell says the work brought a dramatic reduction in maintenance hours, and both companies now gather data from every aircraft to spot failing parts before they ground it.

• V-22 (FSD and EMD): six full-scale development prototypes from 1989, followed by redesigned engineering and manufacturing development aircraft from 1997.
• MV-22B: Marine Corps assault transport for troops, equipment and supplies, operating from amphibious ships or austere bases; replaced the CH-46E and CH-53D. Also used for VIP support by HMX-1. Japan’s Ground Self-Defense Force operates the same Block C standard.
• CV-22B: Air Force Special Operations Command variant with terrain-following radar, extra wing fuel tanks, forward-looking infrared and infrared countermeasures, for long-range infiltration, exfiltration and resupply. Replaced the MH-53 Pave Low.
• CMV-22B: Navy carrier onboard delivery version with enlarged sponson tanks, high-frequency radio and a public-address system; NAVAIR quotes 6,000 lb carried 1,150 nmi.
• HV-22, SV-22, EV-22: proposed Navy rescue, anti-submarine and airborne early warning versions that were studied but not built.

The Osprey’s price has always been part of its story. The Government Accountability Office noted that 1986 plans for nearly 1,000 aircraft at US$37.7 million each, in fiscal 2009 dollars, had become fewer than 500 aircraft at US$93.4 million each, a 148 percent increase in procurement unit cost. Operating cost drew similar scrutiny. Marine aviation leaders said in 2011 that changes in flying procedures had cut about US$3,000 from each flight hour, bringing the figure from roughly US$12,000 to around US$9,000, but earlier GAO work had found it to be more than double the original target.

Readiness is the other half of the equation. In Iraq the three deployed squadrons averaged mission-capable rates of 57 to 68 percent against an 82 percent requirement, and some components lasted less than a tenth of their predicted life. Bell and Boeing have since set up fleet data analysis and nacelle improvement programmes aimed specifically at maintenance hours. Any figure for cost per hour should be read as an estimate that varies with variant, year and accounting method.