PyroCBs: Ominous Clouds and Your Aircraft

In this photo provided by the Oregon Office of State Fire Marshall, flames and smoke rise from the Bootleg fire in southern Oregon on Wednesday, July 14, 2021. Photo credit: John Hendricks/Oregon Office of State Fire Marshal via AP

This is the final part of the series of articles on pyroCBs. Here are the first and second parts.

The “Fire Influence on Regional to Global Environments and Air Quality” (commonly referred to as “FIREX-AQ”) was a comprehensive wildfire smoke investigation with field investigations carried out during the wildfire season of 2019. Mission planning for FIREX-AQ was complex involving hundreds of instruments and scientists from six federal agencies, 22 academic institutions, two foreign governments and numerous private-sector companies that collaborated in a complex logistical operation.

Specially instrumented aircraft were utilized to collect air quality data sampling. The NASA DC-8, the world's largest flying chemistry laboratory, was the flagship of FIREX-AQ.  This modified DC-8 carries a comprehensive suite of chemistry and aerosol instruments to analyze fire emissions in flight and study chemical reactions and transformations in smoke from both Western wildfires and agricultural/land-management burning emissions across the United States.

A NASA ER-2 high-altitude research aircraft was also used. Correlating the in-flight measurements of other aircraft with the ER-2's high-resolution observations will expand the ability of scientists to utilize lower-resolution data provided by satellites to understand what's happening on the ground.  The ER-2 operated from its base in California during this project. 

A modified DHC-6 Twin Otter from NOAA deployed to Boise, ID and other locations across the northwest U.S. in July and August 2019 to collect samples of airborne emissions, photochemistry and nighttime chemistry in coordination with other research platforms. The NOAA Twin Otter focused on the variability of the emissions measuring close to a wildfire for extended time periods; the fast evolution of smoke in the first few hours after emission; and the vertical distribution in the concentration, composition and optical properties of smoke in regionally impacted western valleys.

While the data crunching from the volume of information collected in 2019 will continue for years, specialists at NOAA’s Chemical Sciences Division authored a report titled “The Impact of Wildfires on Climate and Air Quality.” They found that a wide variety of chemical compounds were released by wildland fire. The long list includes greenhouse gases [carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O)] and photochemically reactive compounds [e.g., carbon monoxide (CO), nonmethane volatile organic carbon (NMVOC) and nitrogen oxides (NOx)]. 

Obviously there are threats to human health as well as effects on the climate from this mixture.  For purposes of our study, it certainly brings into question if we should be concerned about the effects of these compounds on the operation of aircraft.

Engines Are Like Vacuum Cleaners

Turbine engines were designed to operate in “normal” air, which is composed of 78% nitrogen, 21% oxygen and 1% other gases. The proper ratio of oxygen is absolutely necessary in the combustion process. If the oxygen concentration in the engine inlet air drops below a certain ratio then the combustion process may no longer be sustainable, resulting in an engine flame-out.   

In contrast, the composition of smoke plumes contrasts significantly from the “normal” airborne environment, both in chemical composition as well as particulate matter. The smaller-sized particulates remain suspended aloft in the atmosphere longer than coarse particles, and therefore disperse farther from the wildfire.   

The Australian Civil Aviation Safety Authority published an airworthiness bulletin (#72-002, dated Sept. 29. 2006) titled  “Engines Operating in a Fire Fighting Environment” advising operators about the multiple effects of smoke particles on turbine engines. To begin with, particles ingested from the smoke can block small-diameter pressure sensing and control lines, leading to erratic fuel metering to the combustion chamber, erratic engine operation and malfunction of accessories.

Secondly, these particles can block fine cooling holes within turbine blades. This results in the loss of cooling air to the turbine blades, exposing blades to temperatures beyond their design parameters.  

Airborne particles will also cause deposition of contaminants inside heat exchanges (e.g. oil cooler) that may block free air flow and result in decreased heat transfer, which will then elevate fluid temperatures. Higher than permitted fluid temperatures result in rapid deterioration of internal engine components.

At night you won’t see this smoke plume so it is entirely possible for a high flying aircraft to fly through a region polluted by a smoke plume. Are the concentrations of smoke plumes at altitude sufficient to cause concern about the effects on turbine engines? Will the high altitude ash particles and other compounds in the smoke plumes affect the cabin environmental system? Will the air filtration system capture these? How soon will these clog up the filters in the air system? These are valid questions that need validated studies.  

Should We Be Concerned?

As promised at the beginning of this article, recent scientific research from subject matter experts working within respected organizations was summarized to give you some of the early insight into the immensity of this newly designated weather event.  

Dr. David Peterson, et al, found that detailed airborne sampling, in combination with ground and spaceborne observations, is therefore essential for improved understanding of pyroCb impacts. (Source: Peterson, David A., et al. “Wildfire-driven thunderstorms cause a volcano-like stratospheric injection of smoke.” Climate and Atmospheric Science, Aug. 20, 2018.)

 “An analysis of the smoke aerosol particles injected into the lower stratosphere from five near-simultaneous intense pyroCBs observed over western North America on Aug. 12, 2017 found that the physical particle properties and chemical evolution of pyroCb smoke in the stratosphere remains highly uncertain. Processing of pyroCb smoke during the lofting process into the stratosphere will change its composition and properties relative to surface or tropospheric smoke plumes. In addition, since pyroCb updrafts begin with strong surface inflow winds in a dry environment, additional aerosol particles such as mineral dust, may also be contributing factors.”  

Should we be concerned in aviation?  The answer is a probable “yes” but that is qualified with, “However, we really don’t know enough right now…..and specifically, how the products of this cloud will effect an aircraft’s systems.”  

The wildland fire fighting community is rightfully concerned about the substantial hazards created by this atmospheric monster. At this point the research on this threat is still in its infancy.  Clearly much more needs to be learned through the scientific process, and then properly translated into terms that are usable for the aviation industry.  It 

Author’s Note:  This is a special thank you to the several aerial fire fighting captains who suggested the importance of bringing attention to this topic and who provided timely feedback as well as information exchange with their weather teams. At this very moment they are engaged in missions along the firelines of the Dixie and Bootleg Fires.  They are facing many more months of grueling fire conditions.  We express our sincerest gratitude and hopes for the safety of all fire fighting teams in these challenging upcoming months.