05/20/2021 A C 450.101-1 Revision A 13 DEMONSTRATED RELIABILITY.
13.1 Required Reliability.
Section 450.101(c)(3) states that one way in which an operator can protect against a high consequence event in uncontrolled areas for each phase of flight is by establishing that the launch or reentry system has sufficient demonstrated reliability as agreed to by the Administrator based on CEC criteria during that phase of flight. The FAA will use the demonstrated reliability and average ground consequence results from fatal accidents involving U.S. civil aviation aircraft with standard airworthiness certificates to establish what constitutes sufficient demonstrated reliability to protect against a high consequence event based on CEC as described in this section. More specifically, sufficiently high demonstrated reliability will be evaluated based on the principles outlined in paragraphs 13.4.1 and 13.4.2 of this AC.
13.2 Demonstrating Reliability.
An applicant should demonstrate reliability based on history of the vehicle being flown, following equivalent practices to type certification for aircraft. Small modifications of a vehicle may be acceptable without being considered a new vehicle. Based on current licensed operations, the FAA anticipates that, initially, only flight phases where an aircraft carries a rocket would be able to meet the demonstrated reliability standard, and the aircraft would need an airworthiness certificate and extensive flight history.
However, it is anticipated that in the future, rocket systems will also develop history and certification practices that will enable commensurate levels of reliability to be demonstrated. Because demonstrated reliability provides an alternative to flight abort when CEC is greater than 1 x 10-3, it is appropriate to assess it consistent with the approach to flight abort and FSS reliability, which depends on CEC with a 1 x 10-2 threshold. For example, a binomial approach would require data from at 87,000 flights without a failure to demonstrate a reliability corresponding to the 8E-6 failure rate (identified in paragraph 13.4.3 of this AC) at the 50% confidence level.9
13.3 Using the Integer Method for Consequence.
13.3.1 The sufficient level of demonstrated reliability to meet § 450.101(c)(3) may be derived by comparing the proposed launch or reentry with an equally hazardous aircraft as measured by CEC. AC 450.115-1, Flight Safety Analysis: Levels of Fidelity describes how a flight safety analysis should be performed for all phases of a flight and contains a discussion of the effective level of de facto risk acceptance. It provides useful guidance as to the reliabilities of different classes of aircraft, the maximum casualty area typical of each class, and the tolerated overflight of regions of various population densities at risk from a failure. AC 450.115-1 introduces the integer method for assessing the required level of fidelity for a flight safety analysis. This method represents the key risk variables of maximum casualty area, maximum population density, reliability, and acceptable risk using integer values. Each of these variables is characterized on a 9 For example, see equation 13.9 in “An Introduction to Reliability and Maintainability Engineering” by Charles E.
Ebeling, Waveland Press, Inc., Long Grove, IL, 2019, ISBN-13: 978-1577666257.
Revision A logarithmic scale. This allows an assessment of the level of fidelity required for the analysis to meet § 450.115(b)(1). This simple method was used to infer a de facto acceptable level of risk based on how several classes of aircraft are allowed to operate (commercial aircraft, general aviation, experimental aircraft, and UAVs). The maximum casualty area and the maximum population density together provide a conservative measure of CEC. Phases of flight for analysis should be defined in accordance with the guidance provided earlier in this AC. The appropriate maximum casualty area is that which is applicable to the phase of flight being analyzed; the maximum population density should, similarly, be assessed based on the flight corridor applicable to the phase of flight being evaluated. When appropriate values for the events being examined are selected, the value of CEC may be estimated as:
= 10(C1+C2−12)CEC Where C1 is the logarithm (base 10) of the approximate maximum population density, in units of people per square mile, and C2 is the logarithm (base 10) of the maximum casualty area, in units of square feet, for the phase of flight.
13.3.2 AC 450.113-1, Flight Safety Analysis: Levels of Fidelity simplifies the task of estimating the maximum population density by providing land use categories that are easily identifiable from a land-use map or aerial photograph. These are correlated with an integer approximation, C1, to the logarithm of the maximum population density.
These integer values are provided in Table 1 of this AC. These are upper estimates of population density of any region. The maximum population density for any whole city is about 75,000 people (less than 1E5), but specific areas (e.g., lower Manhattan)— which aircraft are permitted to fly over—are well above 1E5.
Table 1 — Relationship between Category of Occupancy and Log (Population Density) C1 Categories 6 Major metropolitan area 5 Small city 4 Suburban or small towns 3 Rural 2 Scattered mountain or desert occupancies 1 Notice to keep out only 0 Notice to keep out and either access controlled or surveillance 23
05/20/2021 A C 450.101-1 Revision A
13.4 Existing Aviation Data.
Existing data is available and provides findings necessary to relate several classes of aircraft to consequences and reliability requirements. Reliability is simply the converse of failure probability, so reliability = 100% minus failure probability. Consequences for aircraft are not, however, available in terms of ground casualties. This information is only available in terms of ground fatalities. Conditional expected fatalities, 10 CEF, is therefore used as an equivalent level of safety to compare against CEC, as this is a conservative approach.
13.4.1 Commercial transport aircraft have large casualty areas (up to approximately one million square feet). As a result of rigorous system safety programs and high redundancy of safety critical systems, they are allowed to fly over highly concentrated population centers in large cities. They are allowed to overfly cities and have a demonstrated probability of a failure resulting in a fatal accident per flight of less than 1E-6 (8E-7). These aircraft have a CEF, as measured by fatalities per fatal accident, of approximately 1. This CEF is then compared to the CEC of the applicant’s vehicle for purposes of § 450.101(c)(3). Flight Safety Analysis should protect against high consequence events for purposes of § 450.101(c)(3) by establishing that a launch or reentry vehicle sufficiently demonstrates reliability per the failure probabilities and conditions described in AC 25.1309-1A, System Safety Analysis and Assessment for Part 23 Airplanes.
13.4.2 Fixed wing, non-commercial transport, general aviation aircraft (including business jets) are characterized by a smaller casualty area (typically up to 10,000 square feet).
Their system safety programs are quite rigorous, although they lack the level of redundancy in safety critical systems that commercial transport aircraft have. They are allowed to overfly cities with a demonstrated probability of a failure resulting in a fatal accident per flight of less than 1E-5 (8E-6). These aircraft have a CEF of approximately
0.01 as measured by fatalities per fatal accident. Flight Safety Analyses should protect against high consequence events for purposes of § 450.101(c)(3) by establishing that a launch or reentry vehicle sufficiently demonstrates reliability per the failure probabilities and conditions described in AC 23.1309-1E.
13.4.3 Figure 3 of this AC connects two data points, the estimated failure probability (7.8E-6) and maximum conditional EF (0.01) for Fixed Wing General Aviation and the point with the corresponding values for Commercial Transports (failure probability = 7.8E-7, maximum conditional EF =1.) These points have been connected by a straight line indicating an assumed relationship between allowable failure probability and maximum conditional EC (CEC).
10 CEF represents conditional expected fatalities and is used to measure the mean number of fatalities predicted to occur given an event with a probability of 1. The FAA found that about one ground fatality resulted on average from one hundred fatal accidents involving U.S.-registered aircraft operated under Part 91 between 1984 and 2013 based on NTSB data. A comparison of CEC to CEF is appropriate here because the CEF values cited here are empirical results from aviation accidents, whereas the CEC values used here are the results of physics-based computer simulations for launch and reentry operations. In addition, the differences between aviation and space operations justify some margin in the tolerability of the conditional risks predicted for space transportation operations.
> :'= :.c !1l ..0 0 1E-6 ,._ a.
Q) ,._ :::J !1l LL 1E-7
0.01 Fixed wing general aviation
0.1 Commercial _!, transports 1 Conditional Expected Casaulties 05/20/2021 AC 450.101-1 Revision A Figure-3. Tolerable Failure Probability Per Flight vs CEC
13.4.4 If the estimated CEC falls between the CEF data for Fixed Wing General Aviation and that shown for Commercial Transports, then the tolerable failure probability for that segment may be estimated by interpolating between the failure probability for the two classes of aircraft, in logarithmic space, as illustrated in the graph in Figure 3.
13.5 Hypothetical Example of Demonstrated Reliability.
13.5.1 An applicant is applying for a license to launch its booster. The booster will be airdrop launched from a modified commercial transport aircraft. The booster is 20 feet long with a diameter of 4 feet; the maximum casualty area is estimated to be 80 square feet as tests show that it will not explode on impact. The aircraft will fly out of Mojave Air and Space Port in a northeasterly direction. The flight path will take it between California City to the north and North Edwards and Aerial Acres to the southeast. The planned drop point is approximately eight miles northeast of California City on a heading just east of north. Figure 4 of this AC depicts this region. Figure 4 of this AC also depicts the normal flight path anticipated for launch, which is represented by the black arrow for the carrier aircraft.
13.5.2 California City was assumed to be the highest population density community in the vicinity of the flight corridor. California City was modeled as a suburban region or a small town. As such, Table 1 of this AC provides a population density value (C1) equal to 4 (suburban and small towns). An impact of the carrier aircraft is characterized by 6 (one million square feet casualty area); an impact of the booster is characterized by 2 (eighty square feet). The estimated CEC for the failed aircraft is 10(4+6-12) = 0.01; the estimated CEC for the booster is 10(4+2-12) = 1E-6.
City Aerial Acres Fleta Sanborn Nor1h Edwards B ssen 05/20/2021 AC 450.101-1 Revision A
13.5.3 Based on Figure 3 of this AC, the tolerable failure probability for the carrier would be the same probability of failure associated with large carrier aircraft (8E-7). Specific operations of the aircraft related to the addition of the rocket require additional evaluation, but for this example, let us assume they have been determined to have a probability approximately equivalent to the unmodified aircraft.
13.5.4 Operation of the carrier aircraft in a remote area results in a lower required reliability for the captive carry portion of flight than would be expected purely from the size of the craft. Note that the estimated CEC for the booster is well outside the range of values allowed in Figure 3 of this AC as any extrapolation from the figure is not valid.
Furthermore, the reliability of the booster after release is not assessed to be within the criteria established in Figure 3. Use of the value from Fixed Wing General Aviation would be considered a conservative, reasonable value for the commercial transport carrier aircraft as the estimated CEC is substantially less than that for Fixed Wing General Aviation.
Figure-4. Map of Extended Launch Area 26
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