Understand why aircraft fire-testing standards reference different temperatures, and learn how Fahrenheit, Celsius and calibration requirements influence compliant fire-test procedures and qualification evidence.
Accredited
Scope Available
Programmes
Plus Certified
Expertise
Test Evidence
Aircraft powerplant fire-test documents use both Fahrenheit and Celsius. Frequently encountered values include:
1,850°F
2,000°F
2,150°F
1,010°C
1,093°C
1,100°C
1,177°C
These values are not necessarily inconsistent. Some are exact mathematical conversions. Others are rounded engineering values or requirements defined independently within a particular standard.
This distinction matters. A laboratory should not replace the stated requirement with a preferred rounded value simply because the two figures describe a broadly similar thermal environment. The applicable regulation, standard, customer specification and approved test procedure remain controlling.
FAA powerplant guidance historically refers to severe fire exposure around 2,000°F. ISO 2685 defines its standard flame using 1,100°C ±80°C and an absorbed heat-flux density of 116 kW/m² ±10 kW/m². SAE AS6826 uses a minimum standard-flame temperature of 2,000°F or 1,093°C, together with its prescribed heat-transfer measurement requirements.
An absolute temperature identifies a position on a temperature scale.
To convert Fahrenheit to Celsius:
°C = (°F − 32) × 5/9
| Fahrenheit | Exact Celsius conversion | Practical rounded value |
|---|---|---|
| 1,850°F | 1,010.0°C | 1,010°C |
| 2,000°F | 1,093.3°C | 1,093°C |
| 2,150°F | 1,176.7°C | 1,177°C |
Therefore:
2,000°F = 1,093.3°C
The commonly stated value of 1,093°C is the correctly rounded Celsius conversion.
The reverse conversion is:
°F = (°C × 9/5) + 32
Applying this equation:
1,100°C = 2,012°F
This explains why FAA or SAE documentation may refer to 2,000°F or 1,093°C while ISO 2685 uses 1,100°C. The values describe closely aligned severe fire conditions, but they originate from differently expressed technical requirements.
ISO 2685 defines a standard flame temperature of:
1,100°C ±80°C
This converts to a nominal Fahrenheit temperature of:
2,012°F
The ±80°C tolerance converts as a temperature difference, not as an absolute temperature:
80°C × 9/5 = 144°F
The equivalent Fahrenheit expression is therefore approximately:
2,012°F ±144°F
This is close to the historically familiar FAA expression of 2,000°F ±150°F, but the two statements should not be treated as word-for-word substitutes.
ISO 2685 and FAA or SAE methods may differ in burner selection, calibration terminology, instrumentation, heat-transfer measurement, vibration, specimen arrangement, operating conditions and acceptance criteria. Similar nominal temperatures do not establish complete technical equivalence between test methods.
A temperature increase, reduction, tolerance or range is a temperature difference. It does not represent a position on the Fahrenheit or Celsius scale.
For temperature differences:
Δ°C = Δ°F × 5/9
Δ°F = Δ°C × 9/5
The 32-degree offset is not used.
| Temperature difference | Correct equivalent |
|---|---|
| 150°F | 83.3°C |
| 144°F | 80°C |
| 118.8°F | 66°C |
| 80°C | 144°F |
For example:
150°F × 5/9 = 83.3°C
A tolerance of ±150°F therefore converts to approximately ±83°C, not ±66°C and not ±115°C.
This distinction becomes important when reviewing:
Flame-temperature tolerances
Burner adjustment limits
Calibration acceptance bands
Permitted temperature reductions
Temperature increases
Measurement uncertainty
Test deviations
Applying the absolute-temperature equation to a temperature difference introduces an incorrect 32-degree offset and can materially change the interpreted requirement.
A valid aircraft fire test is not established by flame temperature alone.
Depending on the applicable method, the controlled fire environment may also include:
Heat-transfer rate or heat-flux density
Flame coverage and geometry
Burner-to-specimen distance
Fuel type
Burner orientation
Exposure duration
Airflow
Differential pressure
Fluid pressure, flow and temperature
Electrical loading
Structural or mechanical loading
Vibration
Functional monitoring
Post-test inspection
For example, ISO 2685 uses an absorbed heat-flux density requirement of 116 kW/m² ±10 kW/m². AS6826 uses a prescribed copper-tube apparatus and a minimum heat-transfer rate of 4,500 Btu/h or 1,319 W. These quantities must not be presented as though they are the same measurement.
A burner can achieve the nominal temperature while failing another required calibration parameter. Temperature compliance by itself does not demonstrate that the complete standard fire environment has been produced.
Before converting or comparing any fire-test value, establish what the number represents.
Ask:
Is it an absolute flame temperature?
Is it a temperature tolerance?
Is it a minimum value?
Is it a nominal value?
Is it a temperature reduction or increase?
Is it a calibration acceptance band?
Is it a test-operating limit?
Which standard and revision control the programme?
The applicable route may involve:
ISO 2685
FAA AC 20-135A
Legacy FAA AC 20-135
FAA Powerplant Engineering Report No. 3A
FAA Aircraft Materials Fire Test Handbook methods
SAE AS6826
SAE AS1055
SAE AIR1377
SAE AS4273
RTCA DO-160 Section 26
An OEM or customer-specific qualification procedure
These documents are related, but they are not interchangeable. Regulations define the required safety outcome. Standards, advisory material and approved test procedures provide methods for producing the evidence.
FAA AC 20-135A is the current FAA guidance identified in Resonate Testing’s internal technical material. It recognises SAE AS6826 as an acceptable means of compliance for applicable powerplant fire-protection requirements and cancels AC 20-135 Change 1.
Legacy AC 20-135 terminology and test evidence may remain relevant to existing certification programmes. New programmes should confirm whether AC 20-135A, AS6826, ISO 2685 or another agreed method controls the compliance route.
This article explains temperature-conversion principles. It does not establish that two standards, burner systems or certification routes are technically equivalent.
For an ISO/IEC 17025 accredited laboratory, the critical question is not whether a value is written in Fahrenheit or Celsius.
The laboratory must demonstrate that:
The applicable method and revision have been identified.
Conversions are mathematically correct.
Rounding rules are controlled.
Calibration records are traceable.
Instrumentation is suitable for the required range.
Measurement uncertainty is considered where applicable.
Test procedures use consistent units.
Reports preserve the governing requirement.
Deviations and technical interpretations are documented.
Accreditation claims remain within the laboratory’s confirmed scope.
Where a source document provides both imperial and metric values, the laboratory should preserve the values stated in that document unless an approved procedure defines another approach.
An apparent numerical discrepancy should be investigated through the source document, revision history, certification plan and authority guidance. It should not be resolved by assumption.
Before approving a fire-test procedure:
Identify the controlling regulation, standard and revision.
Determine whether each value is absolute or differential.
Apply the correct conversion formula.
Retain sufficient decimal precision during calculations.
Round only at the final reporting stage.
Record the original value and converted value.
Confirm whether the requirement is nominal, minimum or tolerance-based.
Check temperature and heat-transfer requirements separately.
Verify burner calibration requirements.
Confirm agreement with the customer or certification authority where interpretation remains necessary.
This process reduces avoidable disputes and helps protect the technical validity of the resulting evidence.
Aerospace compliance depends on correct interpretation as well as correct test execution.
Resonate Testing reviews the applicable standard, test configuration, operating conditions, calibration requirements, instrumentation and acceptance criteria before hardware enters the laboratory. The objective is to perform the correct test against the correct requirement and produce a traceable, technically defensible evidence package.
Our powerplant fire-test capability supports components, firewalls, nacelle structures, hose assemblies, fluid systems and electrical hardware, subject to technical review and confirmation of the applicable accredited scope.
Use the contact route to share the requirement, test profile, drawings or specification for review.
Whether you’re looking to contact us for the first time or have another testing requirement, we’d love to hear from you.
