Tiny particles suspended in the air reflect blue light and cause haze. The infrared film is not sensitive to blue light (560 nm and lower) so most atmospheric haze is eliminated. When haze obscured accident site features on routine color aerial photographs, infrared false color prints documented site details with much better contrast resolution.
Documentation and analysis of soil disruption patterns that reflect ground impact events is a primary application of broad spectrum aerial photography. Soil disruption patterns on photographs normal to the initial ground impact area provide information about aircraft configuration, attitude and flight path angle at impact. This information is essential to reconstruction of the accident sequence.
Infrared photographs usually improved terrain contrast, promoting soil disruption pattern recognition. Occasionally soil disruption areas not apparent on routine color prints were obvious on false color infrared prints. Displaced subsoil sprayed over undisturbed surface soil stands out in infrared prints because of variations in moisture content but is not discernible on standard color prints.
Burn patterns from crash related fires are characterized by black soot on both normal color and infrared films. Infrared film is more sensitive than conventional film to variations in soot density which is burn patterns and variations in soot density, broad spectrum photography aids accident sequence reconstruction. Asymmetry of burn patterns may correlate with prevailing winds, aircraft fuel distribution, impact attitude and other factors.
Fuel and other hydrocarbons spilled in vegetated areas quickly produce plant toxicity patterns not seen on normal color films but easily detectable on false color infrared photographs. The liquid hydrocarbons including engine oil, fuel, transmission fluid, and hydraulic fluids decrease infrared reflectance from contacted plants before surface color changes are detectable. Hydrocarbon dispersion in conjunction with burn patterns correlate with prevailing winds, aircraft fuel distribution, impact attitude and other factors in the accident sequence. In one investigation asymmetry of the fuel spill pattern provided evidence suggestion a significant fuel imbalance. Wreckage examination confirmed this impression and the accident investigation board considered fuel imbalance an important factor in the mishap. In other cases, infrared photography along the final flight path documented fuel spills secondary to tree strikes and inflight break-up of the aircraft.
Heat injured vegetation usually has decreased infrared reflectance. Even minor heat damage is easily detectable on infrared photographs. The infrared changes from heat damage are similar to hydrocarbon toxicity by are more focal. Infrared photographs from one accident site demonstrate heat damage to conifers from ejection seat rockets and jet engine exhaust.
Wreckage distribution correlates closely with aircraft break-up and the overall accident sequence. Aerial views of the accident site aid reconstruction of the accident sequence from wreckage distribution patterns and supplement other data including aircrew injury patterns. Aerial perspectives are particularly helpful in accidents with widely scattered wreckage such as inflight break ups and high speed, low angle ground impacts. The enhanced contrast and haze reduction of infrared photography helped locate missing aircraft fragments in several accidents.
Infrared photography is particularly useful for detecting tree strikes. The inner wood of trees appears brilliant white on infrared photographs so tree strike patterns are more explicit on infrared false color prints. Infrared color photographs from several accidents demonstrated small exposed wood surfaces, such as branch tips, associated with rotor blade and ejection seat tree strikes. These features were not visible on ordinary color photographs.
Broad spectrum aerial photography is a useful and practical tool in aircraft accident investigations. Soil disruption patterns, wreckage distribution, burn areas and fuel dispersion patterns often appear in greater contrast and detail in infrared photographs. Analysis of false color infrared photographs may reveal missing aircraft parts and subtle evidence of tree strikes. Special infrared properties allow early detection of foliage changes due to hydrocarbon toxicity and heat damage.
by SSgt Rob Jensen
United States Air Force
4927 B. N Seward Ave
Eielson AFB AK. 99702
Article submitted by the author