Unmanned Aircraft Systems Availability and Continuity (Rev1)

ACP-WGF23/WP09
/
International Civil Aviation Organization
WORKING PAPER / ACP-WGF23/WP09rev1

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

TWENTY THIRD MEETING OF WORKING GROUP F

Cairo, Egypt 21 – 27September 2010

Agenda Item 6: / Any other business

Unmanned Aircraft Systems – Availability and Continuity

(Presented by John Mettrop)

SUMMARY
This paper compares how an instruction from an air traffic controller is translated into an action by an aircraft for both manned and unmanned aircraft. Identifies the additional elements present in the unmanned situation and then looks at how this might be used to identify the minimum acceptable reliability, availability and continuity figures for the Unmanned Aircraft Systems (UAS) and how this compares to figures being considered in RTCA.
ACTION

To note this paper

Introduction
UAS have been operated for some time by various agencies. These operations have been restricted to segregated airspace where entry into a designated volume of airspace is restricted. For UAS operations these volumes can either be a fixed volume such as a range or where a UAS is transiting areas of segregated airspace moving volumes of airspace. Whilst use of fixed volumes of segregated airspace is relatively simple once the volume of airspace is established, moving volumes of airspace require a significant amount of work prior to each mission and hence can only be accommodate on an occasion basis.
As UAS technology develops it is expected that there will be a regular requirement for aircraft using UAS technology to fly in non-segregated airspace. If this is to be achieved then those aircraft will have to act in a similar manner to manned aircraft and hence be effectively transparent to ATC whilst not decreasing the safety of the airspace for other aircraft flying in the same airspace. Therefore unmanned aircraft will have to be able to turn an instruction from an air traffic controller into an action by the aircraft with the same or even high level of reliability as a manned aircraft
Discussion
In the case of a manned aircraft an air traffic controller will issue an instruction, this will be sent via a ground network to a radio station where it will transmitted over a given radio channel to the aircraft. At the aircraft the signal will be received and sent via a cable to the cockpit where the pilot will receive the message, determine the action required and then make the necessary input to the controls which in turn is then transmitted via either a cabled or hydraulic link to the relevant control surface/ engine etc.
In the case of an unmanned system an air traffic controller will issue an instruction, this will be sent via a ground network to a radio station where it will transmitted over a given radio channel to the aircraft. At the aircraft the signal will then be sent via a cable to a second radio where it will be sent either via a satellite or a terrestrial link to the UAS pilot. The pilot will then determine the action necessary and through the systems at his disposal sends a radio signal to the aircraft instructing it to take a certain action. The aircraft will receive this signal and relay that message via a cable to the relevant control surface/ engine etc.
The major difference between the two processes are the links between the unmanned aircraft and the unmanned aircraft pilot in both the forward and return direction and this is highlighted in appendix A to this document.
If it can be assumed that the action of the unmanned aircraft pilot will act in the same way as the pilot in a manned aircraft (this assumption may be true for routine operations but may not be true in an emergency situation) then we can eliminate this element. This would then leave the two direct interfaces being with either the existing air ground radio link or the cable linkage to the actual control surface/engine etc.
Therefore in order for the unmanned aircraft to be as reliable as it’s manned counterpart the continuity/availability of the unmanned aircraft radio systems used to transmit command and control/sense and avoid/ATC relay need to have a negligible impact on the combined failure rate of the air ground radio link and the hydraulic/cable link to the control surfaces.
ICAO document 9869 “Manual on Required Communication Performance” suggest that an RCP 10 figure is required for a controller intervention capability that will support a separation assurance capability of 5 NM (typical separation standard within UK controlled airspace). This equates to a probability/flight hour (p/fl) figure for Continuity of 0.999 and an availability figure of 0.99998.
In an FAA technical note on the “Development of an Electrical Wire Interconnect System Risk Assessment Tool” a failure rate of between 10-9 to 10-10 failures per foot of wire was determined from historical data. This information was confirmed as being consistent with one major manufacturer’s internal failure data. If we assume that the average length of wire is 200 feet then the failure rate of a piece of cable would be between 5x10-6 and 5x10-7 failures per flight hour.
Assuming that the failure in the communications link and those in the wiring system are independent then for a manned aircraft the combined probability per flight hour of the communication/wiring system would be for continuity between 0.998995 and 0.9989995 and for availability between 0.999975 and 0.9999795. These figures assume no redundancy in the wiring system. The addition of redundancy would increase these probabilities.
For the unmanned aircraft radio systems to have a negligible impact then perhaps they should only be responsible for 1 in 100 incidents and hence the minimum probability/flight hour for Continuity would have to be better than 0.99998995 and Availability would have to be better than 0.99999975.
Currently RTCA are considering availability figures of 0.998which is not of the same order of magnitude as those calculated in this paper. Whilst the figures calculated in this paper are not definitive the order of magnitude differences are of concern.
Action by the Meeting
To note this paper.

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ACP-WGF23/WP09