ED Decision 2018/007/R
The target 5-minute capsize probabilities for a rotorcraft certified to
CS 27.801 are:
Certification with ditching provisions:
Fully serviceable emergency flotation system (EFS) – 3 %
Critical flotation compartment failed – 30 %
Certification with emergency flotation provisions:
Fully serviceable emergency flotation system (EFS) – 10 %
Critical flotation compartment failed – no demonstration required
One option available to the rotorcraft designer is to test at the
selected wave height and demonstrate a probability of capsizing no greater
than these values. However, to enhance offshore helicopter safety, some
national aviation authorities (NAAs) have imposed restrictions that prevent
normal operations (i.e. excluding emergencies, search and rescue (SAR), etc.)
over sea conditions that are more severe than those for which performance has
been demonstrated. In such cases, the helicopter may be operationally limited.
These operational restrictions may be avoided by accounting for the
probability of exposure to sea conditions that exceed the selected wave height
by certifying the rotorcraft for a lower probability of capsizing. Since it is
conservatively assumed that the probability of capsizing in sea conditions
that exceed the certified wave height is unity, the lower capsize probability
required to be met is the target value minus the probability of the selected
wave height being exceeded. However, it should also be noted that, in addition
to restricting normal helicopter overwater operations to the demonstrated
capability, i.e. the applicant’s chosen significant wave height limit (Hs(limit)),
an NAA may declare a maximum limit above which all operations will be
suspended due to the difficulty of rescuing persons from the sea in extreme
conditions. There will, therefore, be no operational benefit in certifying a
rotorcraft for sea conditions that exceed the national limits for rescue.
In the following examples, we shall use the three target probabilities
of capsizing without any reduction to avoid operational restrictions. The test
times quoted are full-scale times; to obtain the actual model test run time,
these times should be divided by the square root of the model scale.
Certification with ditching provisions —
fully serviceable EFS
Taking this first case, we need to demonstrate a ≤ 3 % probability of
capsizing with a 95 % confidence. Applying equation (5)(i) above, this can be
achieved with a 499-minute (full-scale time) exposure to the sea condition
without a capsize.
Rearranging this equation, we have:
Alternatively, applying equation (5)(ii) above, the criterion would
also be met if the model were seen to capsize just three times (for example)
in a total 21.5 hours of exposure to the sea condition, or four times (for
example) in a total of 25.5 hours of exposure.
Equation (ii) cannot be readily rearranged to solve Ttest, so the easiest way to solve it is by using a spreadsheet on a trial-and-error method. For the four-capsize case, we find that a 25.5-hour exposure gives a confidence of 0.95.
Certification with ditching provisions —
critical flotation compartment failed
In this case, we need to demonstrate a ≤ 30 % probability of capsizing with a 95 % confidence. This can be achieved with a 50-minute (full-scale time) exposure to the sea condition without a capsize.
As above, the criterion would also be met if the model were seen to
capsize just three times (for example) in a total 2.2 hours of exposure to the
sea condition, or four times (for example) in a total of 2.6 hours of
exposure.
Solving by trial and error in a spreadsheet, we find that a 2.6-hour exposure with no more than four capsizes gives a confidence of 0.95.
Certification with emergency flotation provisions
— fully serviceable EFS
In this case, we need to demonstrate a ≤ 10 % probability of capsizing with a 95 % confidence. By solving the equations as above, this can be achieved with a 150-minute (full-scale time) exposure to the sea condition without a capsize.
As above, the criterion would also be met if the model were seen to
capsize just three times (for example) in a total 6.5 hours of exposure to the
sea condition, or four times (for example) in a total of 7.6 hours of
exposure.
Solving by trial and error in a spreadsheet we find that a 7.6-hour exposure with no more than four capsizes gives a confidence of 0.95.
Certification with ditching provisions —
critical flotation compartment failed
As stated in CS 27.802(c), no
demonstration of capsize resistance is required for the case of the critical float
compartment having failed.
This is because the allowed factor of ten increase in the probability of capsizing, as explained in (a)(3) above, results in a probability of 100 %.
[Amdt
No: 27/5]