Air carrier flight crew reported possibly flying through a thin layer of volcanic ash during cruise. The crew maintained the flight plan buffer distances and submitted a PIREP.
Synopsis
Air carrier flight crew reported possibly flying through a thin layer of volcanic ash during cruise. The crew maintained the flight plan buffer distances and submitted a PIREP.
Narrative
We potentially flew through an extremely thin and sparse layer of volcanic ash (VA). During pre-flight we noticed an unusual route filed on the release and upon further discussion noticed the WHISKEY SIGMET pertinent to volcanic ash. We contacted Dispatch and agreed that the more than 100-mile buffer between us and the affected area; with a greater than 5000 ft. buffer in vertical spacing; was appropriate. During climb-out we didn't see any billowing clouds or notice any off-smells; but simply a slightly darker inversion-layer a couple hundred feet thick. As we climbed through it; we observed that it was more cohesive and darker than typical pollution; it was slightly wispy and grey. During the entire flight we didn't notice any odors of ash or sulfur. After discussing the situation for a while in the air; we submitted a PIREP for potential VA and contacted Dispatch. The Captain closely monitored the engine parameters during the remainder of the flight and noticed no irregularities; but still wrote-it up on the ground.The probable cause was that we assumed with our significant lateral buffer between us and the SIGMET; we would be well clear of any possible effects. On the leg prior; we noticed nothing significant on the descent in; and if we hadn't seen the SIGMET; we would have noticed nothing on departure. We weren't sure it was volcanic ash while flying through it and discussed it for a little while; we made the determination that flying back through it was worse than proceeding on to the destination; meanwhile monitoring normal aircraft performance.Suggestions: Education on what volcanic ash might look like. I had an image in my head of billowing thick white clouds directly from a volcano and not a spread-out thin layer which was hard to distinguish. At night it would have been impossible to see. Even flying through the layer; we weren't sure it was volcanic ash; just that it was different and potentially VA.
Second reporter narrative
During pre-flight preparation and briefing; new SIGMET WHISKEY for Volcanic Ash cloud/remnants in the region was noted; reviewed; and discussed with Dispatcher. Flight had been planned on a slightly different routing to keep a wide buffer around the current SIGMET area (more than 100 miles from SIGMET boundary). During our climb out it was somewhat hazy; but flight visibility was relatively good. It appeared to be a temperature inversion with haze at the lower altitudes; and at the top of the inversion appeared to be a very thin layer of pollutant. As we climbed through the very thin (flat) and sparse layer (estimated a few hundred feet thick); we noticed it was also somewhat patchy and scattered and we began to wonder if it could be VA. We observed the engine instruments closely but noticed no effects. We smelled no odors. Without the context of reported drifting ash clouds; we would've been unconcerned by the sight. However; we were just made aware of a new and closer PIREP and considered that it could be ash. We felt we were above it and continued climbing to an altitude well above the reported tops of the distant VA SIGMET. After discussing for some time; we elected to make a PIREP to ATC of the 'possible VA' as well as to Dispatch.Possible cause was we had only just become aware of SIGMET and volcanic ash moving into the region prior to this flight. Having just flown into the terminal area and not observed any weather issues or phenomenon on arrival; there was little concern about the terminal area weather for departure. I knew our flight planned route would keep us well clear of the SIGMET area and it seemed our Dispatch was being proactive by rerouting us even more than necessary. After the encounter; we were not sure that what we'd seen was ash and were initially reluctant to report it as such and cause undue alarm/disruption. Given how serious a threat VA poses to flight operations; we decided we should report it; and have the aircraft inspected by Maintenance; just in case.Suggestions: If there are active volcanic ash NOTAMS / advisories; first and foremost that Dispatch have the responsibility to suspend flight operations to those areas and to notify crews to avoid those areas by a wide safety margin. There should be remarks in the flight release REMARKS for all flights within a certain radius. Airborne flights must be notified. Night operations should be much more conservative with flight planning as the crew will have no way to detect any ash cloud. Based on this experience I would suggest assuming ash has traveled outside of the SIGMET areas and is not neatly contained within it. For crews: if ash is anywhere near your route; take the extra time during preflight planning to form backup plans (return to origin; stay at low or high alt until well clear; direction to escape; suitable diversion airports; etc.) Be sure to ask Dispatcher for any PIREPS; carry extra contingency and/or alternate fuel for any possible deviations; and if in doubt; delay or refuse the flight! Once airborne; if something looks like it could be an ash cloud and any doubt exists and there is time to take evasive action; avoid it!
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.