GM2 SPA.SET-IMC.105(d)(2) SET-IMC operations approval    

CAA ORS9 Decision No. 1

SAFETY RISK ASSESSMENT FOR A SPECIFIC ROUTE

(a) Introduction

The risk assessment methodology should aim at estimating for a specific route the likelihood of having fatalities due to emergency landing caused by engine failure. Based on the outcome of this risk assessment, the operator may extend the duration of the risk period beyond the maximum allowed duration if no landing site is available within gliding range.

(b) The safety target

The overall concept of SET-IMC operations is based on an engine reliability rate for all causes of 10 per million flight hours, which permits in compliance with SET-IMC requirements an overall fatal accident rate for all causes of 4 per million flight hours.

Based on accident databases, it is considered that the engine failure event does not contribute by more than 33 % to the overall fatal accident rate. Therefore, the purpose of the risk assessment is to ensure that the probability of a fatal accident for a specific flight following engine failure remains below the target fatal accident rate of 1.3 × 10-6.

(c) Methodology

The methodology aims at estimating the likelihood of failing to achieve a safe forced landing in case of engine failure, a safe forced landing being defined as a landing on an area for which it is reasonably expected that no serious injury or fatalities will occur due to the landing even though the aeroplane may suffer extensive damage.

This methodology consists of creating a risk profile for a specific route, including departure, en route and arrival airfield and runway, by splitting the proposed flight into appropriate segments (based on the flight phase or the landing site selected), and by estimating the risk for each segment should the engine fail in one of these segments. This risk profile is considered to be an estimation of the probability of an unsuccessful forced landing if the engine fails during one of the identified segments.

When assessing the risk for each segment, the height of the aeroplane at which the engine failure occurs, the position relative to the departure or destination airfield or to an emergency landing site en route, and the likely ambient conditions (ceiling, visibility, wind and light) should be taken into account, as well as the standard procedures of the operator (e.g. U-turn procedures after take-off, use of synthetic vision, descent path angle for standard descent from cruising altitude, etc.).

The duration of each segment determines the exposure time to the estimated risk. The risk is estimated based on the following calculation:

Segment risk factor = segment exposure time (in s)/3 600 × probability of unsuccessful forced landing in this segment x assumed engine failure rate per flight hour (FH).

By summing up the risks for all individual segments, the cumulative risk for the flight due to engine failure is calculated and converted to risk on a ‘per flight hour’ basis.

This total risk must remain below the target fatal accident rate of 1.3 × 10-6 as under (b) above.

(d) Example of a risk assessment

An example of such a risk assessment is provided below. In any case, this risk assessment is an example designed for a specific flight with specific departure and arrival aerodrome characteristics. It is an example of how to implement this methodology, and all the estimated probabilities used in the table below may not directly apply to any other flight.

The meaning of the different parameters used is further detailed below:

AD/Other: ‘AD’ is ticked whenever only aerodromes are selected as landing sites in the segment concerned. ‘Other’ is ticked if the selected landing sites in the segment concerned are not aerodromes. When a risk period is used by the operator, none of the two boxes (neither ‘AD’ nor ‘Other’) are ticked.

Segment exposure time: this parameter represents the duration of each segment in seconds (s).

Estimated probability of an unsuccessful forced landing if engine fails in the segment: probability of performing in the segment a safe forced landing following engine power loss.

Segment risk factor: risk of an unsuccessful forced landing (because of power loss) per segment (see formula above).

 

 

LANDING SITE

 

Assumed engine failure rate per FH

1,00x10-5

 

 

Segments of flight

Assumed height or height band above ground level (AGL) in ft

 

 

AD

 

 

Other

 

Segment exposure time (in s)

Cumulative flight time from start of take-off to end of segment (in s)

Estimated probability of unsuccessful forced landing if engine fails in this segment

 

Segment risk factor

 

Cumulative risk per flight

 

Comment on estimation of unsuccessful outcome

 

Take-off (T-O) ground roll

 

0 ft

 

X

 

 

20

 

20

 

0.01 %

 

5.56 x 10-12

 

5.56 x 10-12

T-O aborted before being airborne.

Runway long enough to stop the aircraft.

Climb-out

0-50 ft

X

 

8

28

0.10 %

2.22 x 10-11

2.78 x 10-11

Aircraft aborts T-O and lands ahead within runway length available.

 

50-200 ft

X

 

10

38

1.00 %

2.78 x 10-10

3.06 x 10-10

 

 

200-1 100 ft

 

 

 

36

 

74

 

100.00 %

 

1.00 x 10-7

 

1.00 x 10-7

Aircraft has to land ahead outside airfield with little height for manoeuvring

 

1 100-2 000 ft

X

 

36

110

50.00 %

5.00 x 10-8

1.50 x 10-7

U-turn and landing at opposite q-code for magnetic heading of a runway (QFU) possible.

 

2 000-4 000 ft

X

 

80

190

25.00 %

5.56 x 10-8

2.06 x 10-7

Climbing to en route height

4 000-10 000ft

X

X

240

430

5.00 %

3.33 x 10-8

2.39 x 10-7

Aircraft able to operate a glide-in approach.

Cruising: emergency area available

 

≤ 10 000 ft

 

X

 

 

5 400

 

5 830

 

5.00 %

 

7.50 x 10-7

 

9.89 x 10-7

En route cruising time with available landing sites along the route within gliding range.

Cruising: emergency area NOT available

 

≤ 10 000 ft

 

 

 

300

 

6 130

 

100.00 %

 

8.33 x 10-7

 

1.82 x 10-6

En route cruising time without available landing sites within gliding range.

Descent to initial approach fix for instrument flight rules (IFR) approach

 

10 000-4 000 ft on a 4° slope (1 200 ft/min)

 

 

X

 

 

 

300

 

 

6 430

 

 

5.00 %

 

 

4.17 x 10-8

 

 

1.86 x 10-6

 

Descent with available landing sites within gliding range, and destination not reachable.

Aircraft has to descend below the glide approach capability to set up for a normal powered landing from 1 000 ft on a 3° approach path

 

 

4 000-1 000 ft on the approach

 

 

 

X

 

 

150

 

 

6 580

 

 

50.00 %

 

 

2.08 x 10-7

 

 

2.07 x 10-6

 

Aircraft descends below the height needed to maintain a glide approach for reaching the airfield. Therefore, it may land short of airfield if engine fails.

Aircraft descends on a 3°approach path

1 000 -50 ft on approach at 120 kt (600 ft/min)

 

 

 

 

95

 

 

6 675

 

 

100.00 %

 

 

2.64 x 10-7

 

 

2.34 x 10-6

Aircraft assumes 3° glideslope, regained to ensure normal landing. Therefore, it may undershoot the landing field if engine fails at this late stage.

 

Landing

50 ft above threshold until touchdown

 

X

 

 

10

 

6 685

 

5.00 %

 

1.39 x 10-9

 

2.34 x 10-6

Aircraft over runway. Engine is to be idled anyway, but failure, while airborne, may surprise pilot and result in hard landing.

 

Landing ground run

Touchdown to stop

 

 

X

 

 

 

15

 

 

6 700

 

 

0.01 %

 

 

4.17 x 10-12

 

 

2.34 x 10-6

Aircraft on ground. Risk negligible, if engine stops on the example runway (very long) providing that all services are retained.

 

1.26 x 10-6

Risk per flight

 

The following likelihood scale may be used to determine the estimated probability of an unsuccessful forced landing:

Probability in % Description
0 Impossible
0-1 Negligible likelihood/remote possibility
1-10 Possible but not likely
10-35 Moderately likely
35-65 Possible
65-90 Likely
90-99 Almost certain

99-100

 Certain