Composite Risk Index Methodology

This page provides a technical description of the preliminary methodology used to calculate the Composite Risk Index (CRI).

NOTE: The CRI methodology has been improved to also consider specific local operating conditions, airspace size, capacity and/or complexity. For more information please visit Composite Risk Index (CRI) Methodology Update.

The CRI represents a cumulative risk value aggregating all reported, assessed and severity classified safety-related incidents to form an index. This measure of risk exposure is based on probability and severity that considers the human perception of equivalent risk.

Note that CRI can be calculated for any given entity, provider or the State.

Figure 1 shows all components of proposed CRI index.

Composite Risk Index components.

Figure 1: Composite Risk Index components.

Acronyms and terminology

Table 1: Acronyms and terminology
Term Definition
CRI Composite Risk Index
CRInorm Composite Risk Index normalised by flight hours
RI Runway Incursion
UPA Unauthorised Penetration of Airspace
SMI Separation Minima Infringement
TECH ATM Specific / Technical occurrences


As a proxy of safety risk within certain airspace, State, or any Entity, the concept of exposure to risk, based on reported / historical safety information was proposed.

In order to calculate the composite risk, each historical, reported occurrence had to have assigned severity (using Risk Assessment Tool methodology) and probability (calculated from historical safety data).

Modelling can be performed using any source of safety occurrences data, as long as the input satisfies the minimum data requirements.

The CRI calculations are based on the following information:

  • Accidents
  • Operation occurrences (high/medium risk incidents with Severity A to C):
    • runway incursions (RI)
    • separation minima infringements (SMI)
    • unauthorized penetrations of airspace (UPA)
  • Other operational occurrences
  • Technical occurrences (high/medium risk incidents with Severity AA/A to C)

A detailed data description can be found at the end of this page.

To take into account the local conditions within each Entity, exposure data, i.e. the total number of flight hours within each Entity is used as an additional input (CRI normalised results).

The process is visualised in Figure 2.

Composite Risk Index process.

Figure 2: Composite Risk Index process.

Occurrences with no severity classification

All severity unclassified/not assessed events (Severity Category D) are distributed into groups A to E, based on historical distribution (determined using the all available safety data).

The probability of occurrence being assigned to a specific severity category is calculated using historical data, separately for each State/Entity in order to simulate/take into consideration the local conditions.

Figure 3 shows how unassessed occurences (severity D) are distributed into other severity groups.

Distribution of unassessed occurences (severity D) into other severity groups.

Figure 3: Distribution of unassessed occurences (severity D) into other severity groups.

Estimated numbers of occurrences

The formula how the total numbers of specific type of occurrence were determined using the distribution based on probability of unclassified occurrences is presented below.

\[ \text{No_occ_est}_i = ((\text{No_D}_i * P_i) + \text{No_occ}_i) \]


estimated number of safety occurrence type \(i\)
number of reported occurrences of Severity Classification D (not determined) for occurrence type \(i\)
probability of safety occurrence type \(i\)
number of reported safety occurrences of type \(i\)

The probability of each type of occurrence is calculated using the simple principle:

\[ P_i = \frac{\text{No_occ}_i}{\text{Total No}_{{occ}_i}} \]

number of reported safety occurrences type \(i\) in a year
\(\text{Total No}_{{occ}_i}\)
total number of reported safety occurrences type \(i\) in a group in all years

CRI calculation using human perception of risk

Using the estimated total number of occurrences for each type and adding to it the human perception of their risk, it is possible to calculate the CRI for each Entity separately. The simple formula to calculate the CRI is presented below.

\[\begin{align} \text{CRI}_i & = \text{No}_{\text{occ_est_i}} * w_i \\ \text{CRI} & = \sum_{i}^{} \frac{\text{CRI}_i}{\text{Total No}_{{occ}}} \end{align}\]


CRI index for occurrence type \(i\)
weight (based on severity and human perception) assigned to specific type of safety occurrence
\(\text{Total No}_{{occ}}\)
total number of all occurrences in a year

In the formula above the Weights added to each equation represent the additional human perception of the risk for the specific event so that the CRI can at the end consider the human perception of equivalent risk.

Introduction of Weights to express the severity of the event allows their description in non-monetary terms which have a meaningful and understandable explanation in human perception.

Each weight value for specific Severity category is determined by using the optimisation technique, with the aim to select a combination of weights that will not disturb the computation of the CRI from year to year if significant changes in reporting are introduced. In other words, the optimisation will find which combination of weights result in the lowest standard deviation of CRI values between the years for each Entity.

Due to a large number of variables involved and the enormous number of combinations possible, the optimisation and selection of the Weights combination is done in stages:

  1. Selection of Weights for accident, all OPS occurrences and TECH occurrences

  2. Selection of Weights for OPS occurrences based on their type (RI, SMI, UPA, OTHER), taking into consideration overall OPS Weight determined in Step 1

  3. Selection of Weights for OPS and TECH occurrences based on their severity (AA/A, B, C), taking into consideration overall OPS Weight determined in Step 2

In addition, each type of weight selection has predefined weight ranges, to allow for an incremental Severity classification order, based on human perception of risk (from accident to Severity C incident, i.e. from high risk to low risk). In other words, each range has an expectation value associated with it. See further details in the methodology report (Commission 2019)

CRI normalisation

To allow the applicability of the CRI to airspaces with different traffic levels, the CRI is normalised by flight hours for each Entity. The CRInorm is calculated based on the following formula:

CRInorm =CRI/(Number of Flight Hours)


For CRI calculation, each historical, reported occurrence besides classification, has to have assigned severity and probability.

Modelling of Weights can be performed using any source of safety occurrences data, as long as the input satisfies the minimum data requirements as in the table below. Note, that safety data, should include occurrence severity, reported and calculated using severity classification of the Risk Assessment Methodology.

Variable Description
Entity the name of Entity within State, ANSP, or department
accident safety occurrence where: a person is fatally or seriously injured; the aircraft sustains damage or structural failure; the aircraft is missing or is completely inaccessible”
total_no_of_ops_occurrences the sum of safety occurrences (incidents) related to OPS
ri_a runway incursions severity A
smi_a separation minima infringments severity A
upa_a unauthorised penetration of airspace severity A
ri_b runway incursions severity B
smi_b separation minima infringments severity B
upa_b unauthorised penetration of airspace severity B
ri_c runway incursions severity C
smi_c separation minima infringments severity C
upa_c unauthorised penetration of airspace severity C
ri_d runway incursions severity D
smi_d separation minima infringments severity D
upa_d unauthorised penetration of airspace severity D
ri_e runway incursions severity E
smi_e separation minima infringments severity E
upa_e unauthorised penetration of airspace severity E
total_no_of_tech_occurrences the sum of safety occurrences related to technical reasons
t_aa technical occurrences severity AA
t_a techechnical occurrences severity A
t_b techchnical occurrences severity B
t_c techechnical occurrences severity C
t_d techechnical occurrences severity D
t_e techechnical occurrences severity E
total_number_of_all_reported_occurrences sum of all OPS and TECH occurences
year year
flight_hours sum of flight hours in Entity for year n


The classification scheme for safety occurrences in ATM specifies six severity categories for ATM related occurrences impacting the safe operations of the aircraft (as per ESARR2).

Severity Description
A serious incident
B major incident
C significant incident
D not determined
E no safety effect


The RAT classification scheme specifies five qualitative frequency categories (repeatability). However, these values are not commonly reported through the AST. Each State/Entity in principle should develop their own quantitative boundaries, which should consider national traffic volumes and specific operating conditions of the national ATM system.

Therefore, the occurrence probability is calculated using historical data from all available years separately for each Entity in order to simulate/take into consideration the local conditions.

Further Information

For further information on the preliminary methodology please refer also to the report explaining the development of the methodology (Commission 2019).

For more detailed information on the updated methodology, please see the PRC Technical Note explaining the further development of the methodology (Commission 2020). The technical note explains several statistical methods used to model index weights, overall computation, logic behind it, its use and limitations, and lastly areas of further improvement and expansion.

Working example

Script calculating the CRI using the explained methodology and sample data are available for testing purposes [here] crizip.

Please feel free to use sample data provided to get familiar with the methodology.

User guide how to set up the environment can be found at ‘README.pdf’ file within repository.


Commission, Performance Review. 2019. “Composite Risk Index (CRI) - Methodology.” EUROCONTROL.
———. 2020. “Updated Composite Risk Index (CRI) - Methodology.” EUROCONTROL.