For low altitude (13-15km) deployment, existing aircraft might be used, but these planes would still require retrofitting with sulfur storage and distribution equipment, and a fleet of such planes would need to be procured. The certified ceiling of widebody aircraft (~13km) is potentially too low though they may be able to be coaxed to higher altitudes; business jets can certainly get higher, but then the number of aircraft that need to be obtained and modified is much larger. The degree of difficulty and time required to do this is not well yet constrained. It is important to note that this uncertainty encompasses field trial aircraft and hardware, which is similar but likely more pressing.
Metric
Time to modify a fleet capable of delivering ~1Mt/yr payload to 14-15km is over 5 years from first large-scale funding.
Uncertainty
No studies to date with any detailed analysis. Degree of uncertainty depends on the desired altitude threshold; getting to 13km is likely relatively straightforward, but 14 or 15km may not be feasible with widebodies and may thus lead to other challenges with larger fleet sizes. Note that we could have chosen to phrase this to state the metric as "to an altitude where the required injection rates are not more than a factor of two lower than a 30N/30S 20km strategy", but we chose to isolate the engineering challenge from the climate one uncertainty. Although Smith et al. (2024) estimate 3 years for the actual retrofitting, most of the 10 years they quote to produce a ready fleet is taken up by procuring aircraft. So perhaps this is actually only 'medium' consequential, since even doubling this retrofitting time would not hugely change the overall timeline.
Decision relevance
If timelines are significantly longer than assumed, then we wouldn't have the capacity to start even early deployment as early as currently modeled scenarios begin (i.e. in the 2030s). This would also limit ability to conduct experiments to understand aerosol microphysics.