Guidance to determine the criticality of failure conditions

Effective Date: 1 December 2006

OPI / Telephone: DTAES 3 - 819-939-4810

Reference: TAM Part 5, Chapter 1, Section 1 

1.    Purpose

1.1.   This Technical Airworthiness Authority (TAA) Advisory provides clarification and guidance regarding the selection of the appropriate Hazard Severity Category in a Record of Airworthiness Risk Management (RARM).

1.2.   This TAA Advisory is not mandatory nor does it constitute a regulation. It describes a means acceptable to the TAA, but is not the only means to demonstrate compliance with the regulation(s). If you elect to use this TAA Advisory, then all the important aspects of it must be followed.

2.    Applicability

2.1.   The TAA Advisory is applicable to organizations involved with RARM preparation and staffing and involved with risk management activities.

3.    Related Material

3.1.   Definitions:

  1. Operational / flight phase: Take-off, cruise, standard tactical manoeuvres and landing; and
  2. Failure Condition Classifications: Failure Conditions may be classified according to the severity of their effects into one of the following categories:

(1)   Catastrophic failure: Two civilian regulator definitions and their sources are provided as follows:

(a)   Failure Conditions which would result in multiple fatalities to occupants, fatalities or incapacitation to the flight crew, or result in loss of rotorcraft (AC 29.1309 – part of AC 29.2C Change 1);

(b)   Failure Conditions that are expected to (would) result in multiple fatalities of the occupants, or incapacitation or fatal injury to a flight crewmember normally with the loss of the airplane (AC 23.1309-1C); 



  1. The phrase “are expected to result” is not intended to require 100 percent certainty that the effects will always be catastrophic. Conversely, just because the effects of a given failure, or combination of failures, could conceivably be catastrophic in extreme circumstances, it is not intended to imply that the failure condition will necessarily be considered catastrophic.
  2. The term “Catastrophic” was defined in previous versions of the TAM rule and the advisory material as a "Failure Condition that would prevent continued safe flight and landing"

(2)   Hazardous/Severe-Major: Failure conditions, which would reduce the capability of the rotorcraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be: 

(a)   A large reduction in safety margins or functional capabilities;

(b)   Physical distress or excessive workload such that the flight crew’s ability is impaired to where they could not be relied on to perform their tasks accurately or completely; or

(c)   Possible serious or fatal injury to a passenger or a cabin crew member, excluding the flight crew. 


Hazardous/Severe-Major failure conditions can include events that are manageable by the crew by use of proper procedures, which, if not implemented correctly or in a timely manner, may result in a ‘Catastrophic’ event.

(3)   Major: Failure conditions which would reduce the capability of the aircraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be, for example, a significant reduction in safety margins or functional capabilities, a significant increase in crew work load or in conditions impairing crew efficiency, physical distress to occupants, possibly including injuries, or physical discomfort to the flight crew.

3.2.    Regulatory References:

3.2.1.   Technical Airworthiness Manual (TAM), part 5, chapter 1, section 2 – Technical Airworthiness Risk Management Rules and Standards.

3.2.2.   Industry documents:

  1. Federal Aviation Administration (FAA) - Advisory Circular (AC) 29.1309 - Equipment, System, and Installations - part of AC 29.2C Change 1 (Certification of Transport Category Rotorcraft); and
  2. AC 23.1309-1C – Equipment, Safety and Installations in Part 23: Airplanes.

4.     Discussion

4.1.     In addition to the definitions provided in the TAM (Table – Hazard severity), further research and experience has yielded some additional material that may be useful in determining the appropriate category to describe a potential risk condition. This material will be especially useful in discriminating between the ‘Catastrophic’ and ‘Hazardous/Severe Major’ categories.

4.2.     If a ‘Catastrophic’ event is manageable by the crew by use of proper Flight Manual Procedures (FMP) when implemented in timely manner, the event should be reduced to ‘Hazardous’.

4.3.     If the failure condition would be ‘Catastrophic’ only under unusual circumstances, it should not be classified as ‘Catastrophic’ failure.

4.4.     During the failure condition classification, the worst credible cases but not the worst conceivable cases should be considered. Similarly, only the average pilot’s capability, not the top or bottom 10%, should be considered. Also keep in mind that severity for a given hazard varies as a function of operational phases, so the phase during which the failure has the highest criticality should be determined.

4.5.     There are three factors to consider when determining the criticality of a failure condition. The three factors are:

  1. time to detection (when failure is detected);
  2. manner in which a failure is detected (how a failure is detected):

(1)   If and how the failure would be detected, important when there is no indication to the pilot;

(2)   Determine how long it may take the pilot to notice that something has failed;

(3)   Determine if there is a cockpit procedure that might help to notice the failure; and

(4)   If the failure is annunciated, determine if there is any FMP to deal with the failure.

  1. flight phase in which the failure has the highest criticality (when a failure is detected).

4.6.     The effect could be determined at the system level, on the other systems, and at the aircraft level. When determining the effect of failure, always consider the typical and the worse credible case conditions. To determine the effect of the failure, evaluate if:

  1. the corrective action (procedure) is followed as per the FMP, if there is no procedure in the flight manual, then determine if this corrective action should be included as a procedure in the flight manual;
  2. the impact of normal procedures followed by the pilot if the failure is not announced but is present; and
  3. there is any likely incorrect action that may be followed due to misinterpretation of the failure. Then determine the effect of the likely incorrect action taken by the pilot.

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