The police helicopter service in England and Wales fails to get air support to thousands of incidents before they have ended and needs "urgent reform", a report says.
The HM Inspector of Constabulary said helicopter flying hours had nearly halved since 2009, and 24,873 missions were cancelled mid-flight in 2016.
The report also said the cost of flight hours had more than doubled since 2009.
It said some forces used drones instead of the National Police Air Service.
HM Inspector of Constabulary Matt Parr said that "urgent reform is needed" to NPAS, which was set up in 2008, and that "there was concern amongst a lot of chief constables".
"If we go on as we are I think we'll get chief constables and PCCs [police crime commissioners] increasingly dissatisfied with the service," he said.
"It's got to be quite urgent reform and… wide-ranging reform."
He said that aviation support for police forces was developing, and the use of "drones and fixed-wing aviation" was an alternative to the current system.
Since the inception of NPAS, the number of stations has reduced from 31 to 15 and the number of helicopters has been reduced from 33 to 19.
The report said the result was that response times for helicopters "vary wildly" across England and Wales.
Forces can call upon NPAS to assist with operations, but usage varies across different forces.
The worst response time was for Cumbria Constabulary, which on average would wait over an hour for a helicopter to arrive to an operation.
The best response times can be found in London, where the average response time to Metropolitan Police calls was 10-and-a-half minutes.
The cost of helicopter support has led some forces to invest in drones.
Surrey and Sussex Police have jointly spent £300,000 on five unmanned aircraft, while Durham Constabulary recently bought a drone for £1,450.
Chairman of NPAS Mark Burns-Williamson said the service "need[s] to consider the next stage of development" and explore "how drones can impact on the wider service".