Oil Fumes, Flight Safety, and the NTSB
2. Materials and Methods
3.1. Accident Reports
3.2. Description of the TPE331 Engine and Seal Assembly
3.3. Bleed Air Testing
3.3.1. Primary Bleed Air Testing Conditions
3.3.2. Additional Engine Bleed Air Testing
4.1. Accident Reports
4.2. Bearing and Seal Failures in the TPE331 Engine
4.3. Bleed Air Testing
4.5. Exposure Control Measures
- For current aircraft that are equipped with bleed air systems, engineering control measures such as sensors and filters should be mandated to prevent inflight exposure to fumes and, thus, improve flight safety. For new aircraft types, non-bleed air supply systems should be standard.
- Given the flight safety implications, all crewmembers should be trained to recognize and respond to the presence of bleed air contaminants .
- The NTSB should reopen the 1984 report and update the conclusions based on what was known then about oil fumes and flight safety, and considering the data and reports published since then. The NTSB should also issue recommendations to the FAA on necessary actions to minimize the flight safety impacts of exposure to oil fumes.
- Until suitably protective measures are implemented fleet-wide, the NTSB should consider pilot impairment from bleed-sourced fumes as a potential casual factor in future aircraft accident investigations. This is particularly important when an accident includes either relevant maintenance history or a pilot’s failure to communicate, for example.
- If only to avoid the appearance of bias, future investigations should be independent of all commercial interests. Even though it is not unusual for the NTSB to include the FAA and manufacturers in accident investigations, both the poorly conceived design of the air sampling trials and the sweeping conclusions, suggest that commercial conflicts influenced this project.
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
|CAMI||United States Civil Aeromedical Institute|
|CFR||United States Code of Federal Regulations|
|CS||Certification Specification (issued by EASA)|
|EASA||European Union Aviation Safety Agency|
|FAA||United States Federal Aviation Administration (issued by the FAA)|
|FAR||United States Federal Aviation Regulation|
|lb/hr||pounds per hour|
|NTSB||United States National Transportation Safety Board|
|psia||pound-force per square inch absolute (relative to zero or absolute vacuum)|
|psig||pound-force per square inch gauge (relative to atmospheric pressure; at sea level 14.7psi)|
|rpm||revolution per minute|
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|Location||Aircraft Type||Registration No.|
|1||1979-08-03||Hays, Kansas||Mitsubishi MU-2B||N208MA|
|2||1979-11-01||Nashville, Tennessee||Mitsubishi MU-2F||N8730|
|3||1979-12-21||Provo, Utah||Mitsubishi MU-2B-20||N2-OBR|
|4||1980-01-11||Atlantic Ocean||Cessna 441||N441NC|
|5||1980-02-14||Near Houston, Texas||Mitsubishi MU_2B-35||N346MA|
|6||1980-02-23||New Orleans, Louisiana||Mitsubishi MU-2-40||N962MA|
|7||1980-12-06||Ramsey, Minnesota||Mitsubishi MU-2-40||N969MA|
|8||1980-12-15||Richmond, Indiana||Mitsubishi MU-2B-30||N93UM|
|9||1981-01-07||Burns, Oregon||Aero Commander 690B||N81521|
|10||1981-04-22||Alpena, Michigan||Mitsubishi MU-2B-20||N9JS|
|Crash No.||NTSB Report No.||Flight Phase, Time of Day, Weather||Pilot Age (Years), Flight Time (h)||Probable Causes/Factors (per NTSB)||No. Deaths ***|
|1 **||MKC79FA046||cruise, night, “clear”||34;|
|Forward main shaft bearing failed; improper inflight decisions; failed to obtain or maintain flying speed.||7|
|2||IAD80FA007||descent, night, three-mile visibility, dry, light fog||PIC age redacted; 1498.|
Copilot age redacted;
|Pilot-in-command (PIC) misjudged the distance and altitude on final approach; inadequate supervision of flight; failed to use checklist.||5|
|3||DEN80FA012||approach, night, “visibility was around one mile”||46;|
|Inadequate pre-flight preparation or planning; improper Instrument Flight Rules (IFR) operation; misjudged altitude; Pilot could not find runway; aircraft came to rest in water.||2|
|4 *||IAD80AA018||descent, night, check||47;|
|Miscellaneous, undetermined; unable to obtain a response from crew; uncontrolled descent; aircraft came to rest in water.||3|
|5||FTW80FA042||approach, evening, two-mile visibility, rain showers||45;|
|Improper IFR operation; altimeter setting incorrect.||4|
|6 **||FTW80FA048||approach, morning, 0.25-mile visibility, fog||51;|
hrs. not reported
|Improper IFR operation; crashed into water.||7|
|7 **||CHI81FA010||approach, afternoon, “good” weather||54;|
|Failed to obtain/maintain speed; icing conditions, including sleet, frozen rain, etc.||5|
|8||CHI81FEG03||approach, night, dense fog||59;|
|Improper IFR operation; icing conditions, including sleet, frozen rain, etc.||0|
|9 **||SEA81FA015||approach, afternoon, overcast, 15-mile visibility, wind calm||32;|
|Miscellaneous/undetermined; uncontrolled collision with ground/water.||2|
|10||CHI81FA051||approach, night, two-mile visibility, light rain, fog||53;|
|Improper IFR operation; crash on final approach, 1.6 miles short of runway; cause unknown.||3|
|Test No.||Carbon Seal Intact|
|Oil Added to|
|Glass Filter in Bleed Air Line?||Reflects Potential Onboard|
|3a–3f||intact||yes; 2–12 lb/hr||yes||no|
|5b||missing||yes; “dirty start”||yes||no|
|14 CFR § 25.831(b) * “Ventilation”||Design||“The aircraft air supply system must be designed to ensure that crew and passenger compartment air [is] free from harmful or hazardous concentrations of gases or vapors.”|
|14 CFR § 25.831(b)(1)“Ventilation”||Design||The carbon monoxide concentration in the cabin and flight deck supply air must not exceed 50 ppm.|
|14 CFR § 25.1309(c) “Equipment, systems, and installation”||Design||“Warning information must be provided to alert the crew to unsafe system operating conditions, and to enable them to take appropriate corrective action. Systems, controls, and associated monitoring and warning means must be designed to minimize crew errors which could create additional hazards.”|
|14 CFR § 121.703(a)(5) “Service difficulty reports”||Operation||Airlines “shall report the occurrence or detection of each failure, malfunction, or defect concerning…[a] aircraft component that causes accumulation or circulation of smoke, vapor, or toxic or noxious fumes in the crew compartment or passenger cabin during flight.”|
|14 CFR § 121.705“Mechanical interruption summary report”||Operation||Airlines shall report each “interruption to a scheduled flight,” such as a diversion, cancellation, or tail swap, caused by known or suspected mechanical difficulties or malfunctions that are not required to be reported under the 14 CFR § 121.703.|
|FAA Order 8020.11D (Chapter 6 and FAA Form 8020-23)||Operation||Airlines shall report accidents and occurrences which are associated with the operation of an aircraft and affect (or could affect) the safety of operation, including smoke/fumes.|
|Crash No.||Description of Crash Conditions|
|1||The NTSB report describes how, when the pilot was en route, at what may have been top of climb, the pilot “reported a loss of oil pressure in the right engine.” At the same time, “the pilot reported smoke and fumes in the cabin.” A few minutes later, the pilot reported that he had shut down the right engine. He then stopped communicating with air traffic control and crashed into a field without putting the landing gear down (per witness reports). The subsequent teardown of the right engine “revealed that the forward main shaft bearing had failed.”|
|6||The NTSB report describes how, on approach, the pilot stopped responding to air traffic control. The aircraft descended into a lake and “all aboard perished.” The report goes on to say that “[d]uring the course of the investigation, it was reported by various persons that the aircraft had a history of smoke, fumes, and carbon monoxide collecting in the cockpit and cabin area...Examination of the interior of the subject aircraft environmental control system revealed an oily residue in the portion which is supplied by the left engine [with a history of a cracked carbon seal] while that supplied by the right engine was clean.”|
|7||The NTSB report describes how, on approach, the pilot did not respond to air traffic control instructions. Witness reports described how the aircraft came out of the clouds spinning with the nose down and crashed into a field. In the engine tear down report, there were references to “black dirt deposits found throughout the entire gas path of the engine… [including] the bleed air ports…” The NTSB report stated that the pilot had failed to obtain/maintain speed and the cause of the crash was unknown.|
|9||The NTSB report describes how ground witnesses observed the aircraft flying away from the airport maneuvering erratically. The aircraft crossed a highway, pitched up steeply, fell, and crashed. The engine tear down was “to determine the condition of the carbon seal and bearing located between the compressor section and gearbox section.” The report concluded that “one carbon seal, due to the discoloring of the oil slinger, was leaking some oil past the face.“ The report also noted that “Bonneville Power maintenance personnel felt the amount of leakage was insignificant,” but the basis for their claim and their qualification to make it were not provided.|
|Trial No.||Description of Sampling Conditions|
|1–2||These two trials were intended to characterize background levels of contaminants in the bleed air stream. The engine compressor carbon seal was intact, and a glass wool filter was installed in the bleed line. Moreover, oil was not purposefully injected into the bleed air stream.|
|3a–3f||These six trials were intended to characterize levels of oil-based contaminants in the bleed air stream through an intact engine compressor carbon seal and a glass wool filter. Oil was injected into the bleed air stream for 90 min at a rate of 2–12 pounds per hour while the bleed air was sampled for carbon monoxide, carbon dioxide, nitrous oxides, and hydrocarbons.|
|3g||This trial was intended to characterize levels of oil-based contaminants in the bleed air stream through an intact engine compressor carbon seal but without a glass wool filter in the bleed stream. The tester injected oil for 15 min, during which time the same gaseous bleed air measurements were made.|
|4||This trial was intended to simulate the potential for worst-case bleed air contamination by removing the engine compressor carbon seal. Of note, though, a glass wool filter was installed in the sampling line and no oil was injected.|
|5a||During this trial, the tester did not collect measurements but ran the engine for 10 min and then shut it down to prepare for a “dirty start” in the following test.|
|5b||During this trial, the engine underwent a simulated “dirty start” which involved internal oil ingestion. The tester measured the concentrations of carbon monoxide, carbon dioxide, nitrous oxides, and hydrocarbons for the first 10 min after the engine was started.|
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Anderson, J.; Scholz, D. Oil Fumes, Flight Safety, and the NTSB. Aerospace 2021, 8, 389. https://doi.org/10.3390/aerospace8120389
Anderson J, Scholz D. Oil Fumes, Flight Safety, and the NTSB. Aerospace. 2021; 8(12):389. https://doi.org/10.3390/aerospace8120389Chicago/Turabian Style
Anderson, Judith, and Dieter Scholz. 2021. "Oil Fumes, Flight Safety, and the NTSB" Aerospace 8, no. 12: 389. https://doi.org/10.3390/aerospace8120389