US safety regulators have rejected a request by Bombardier to delay implementation of a software fix on avionics which could otherwise generate misleading position data.

Concern emerged after several instances in which air traffic control observed "coasting"– or extrapolated stale data – from automatic dependent surveillance (ADS-B) systems, says the US FAA.

The FAA found that the collision-avoidance system altitude as well as other surveillance data, such as that for Mode-S operations, are affected by the issue.

Misleading position data from the aircraft could present a collision risk, it says, by adversely affecting the collision-avoidance system.

"The extrapolation of the data occurs with no warning to the crew," it adds, in a directive mandating software updates to the system processing unit. The directive takes effect on 20 December.

It affects Rockwell Collins units on various airframes, including the Bombardier CSeries and CRJ, and the Embraer Legacy.Bombardier had requested postponing the effective date until 1 January 2020, arguing that the risk was not clear because air traffic control cannot use ADS-B as a primary source until 2020.

But the FAA rejects this request, pointing out that aircraft could accumulate an additional 5,000h beyond the 750h time-in-service compliance deadline in the directive.

The FAA says its risk assessment "does not support" such an increase in the compliance time, adding that stale Mode-S altitude data interferes with collision-avoidance operation, and that the unsafe condition exists irrespective of ADS-B.

It also says that ADS-B is the primary source of data for traffic separation at all en route centres and over 60% of US terminal facilities.

But the FAA similarly declined to shorten the compliance deadline, after pilots' association ALPA expressed concern that the affected equipment is fitted to aircraft operating in high-density airspace. The FAA says it expects the aircraft with heaviest usage will need to be compliant within four months of the directive.

"Reducing the compliance time would create an additional burden not supported by the risk assessments," it adds.