SAE Event Parameters
On the Calculation Data page for an SAE event, properties are listed under three headings: Failure properties, Maintenance properties, and Failure rate adjustments. No properties are available under the Maintenance properties heading.
|
Property
|
Description
|
||
|---|---|---|---|
|
Failure properties
The properties under this heading specify the failure data to use for calculating results.
|
|||
|
Model
|
The model to use. The selection you make here determines the other properties beneath under this heading. It also determines the properties that are shown and/or available under the Failure rate adjustments heading. The model choices for SAE events are:
• Constant probability. When selected, Probability is shown. The availability is equal to the constant probability value that you enter. You can enter any value from 0 to 1. The default is 0. When you enter a value other than 0, the event is not latent, which means that the item is known to be operating at the beginning of each flight.
• Failure rate/MTBF. When selected, you must select Failure Rate (default) or MTBF to indicate what type of value you are entering in the field to the right. You then enter a value greater than 0. For an MTBF value, you must also select Hours (default) or Minutes to indicate the units.
• Average unavailability. When selected, you must select Failure Rate (default) or MTBF to indicate what type of value you are entering in the field to the right. You then enter a value greater than 0. For an MTBF value, you must also select Hours (default) or Minutes to indicate the units. When this model is selected, the Latency page is unavailable. Additionally, Use Approximate Formula is selected under the Failure rate adjustments heading.
|
||
|
Distribution
|
The distribution to use to specify the uncertainty in the parameter. Choices for the Constant probability model are: Fixed, Beta, and Uniform. Choices for all other models are: Fixed, Lognormal, Normal, Gamma, and Uniform.
|
||
|
Standard Deviation
|
If you want to use a standard deviation for the distribution, the value to use. The default is 0. When Fixed is the distribution, this field is unavailable. The default of 0 is used.
|
||
|
Failure Sequence
|
This property is unavailable for SAE events.
|
||
|
Failure rate adjustments
The properties under this heading specify data to use to adjust the failure rate calculated for the event. Availability depends on the model.
|
|||
|
Exposure Time
|
The ‘at risk’ time of the item per flight, which is expressed in hours. The default value is equal to the flight duration. The status of the Latent Failures checkbox on the Latency page determines how exposure time is used.
• When this checkbox is selected, the exposure time is equal to the portion of the time that the item is used per flight. The actual ‘at risk’ time can vary for each flight. The ‘at risk’ time is determined based on the exposure time and other properties specified on the Latency page.
• When this checkbox is cleared, the item is known to be working at the beginning of each flight, and the exposure time is the same for each flight. If the item is used only in certain phases of the flight, then the exposure time is less than the flight duration. However, in some cases, you might want to specify the exposure time to account for latent failures without specifying the factors that automatically determine the exposure time. To accomplish this, you would enter an exposure time that is greater than the flight time. This is applicable when you want to explicitly model the worst case scenario. You would leave the Latent Failures checkbox cleared, even though the item under consideration is subject to latent failures.
|
||
|
Use Approximate Formula
|
Indicates whether to calculate exact or approximate failure probability results. SAE ARP4761 and ISO 26262 supply the same approximate formula. Use of this formula is suggested when λt is less than or equal to 0.1. Consequently, you might want to select this checkbox for small λt values.
• When Constant probability is the model, this checkbox is unavailable.
• When Failure rate/MTBF is the model, you might want to select this checkbox for small λt values.
• When Average unavailability is the model, this checkbox is selected and unavailable. This is because this model requires approximate calculation. All properties on the Latency page are also unavailable.
|
||
