Assessing Equivalent Level of Safety Argument

Overview

In this scenario you are a member of the MAA evaluating the certification evidence provided by the Ghost DT to contribute to the TCR.  

This Type Certification Review will require you to analyse and assess an ELoS argument. This will give you some valuable insight into the MAA’s role as the independent regulator during this stage of the MACP.

A scenario for a non-compliance with the Ghost RG Mk1 RPAS is detailed below. The DT have provided an MCRI with an ELoS proposed.

In your role as MAA, you were consulted by the DT when they generated the ELOS and have seen the evidence that is detailed below. You are to assess the ELoS the TAA submitted in the form of an MCRI and decide whether the ELoS argument merits agreement and an MSF.

You are to brief the other syndicates using the ppt slide template provided and explain to them the scenario and whether you (as the MAA) would accept the ELOS and provide an ESF or reject the ELoS argument, giving reasons why. You have a maximum of 10 minutes for your brief.

Ghost RG Mk1 ELOS

The Ghost RG Mk 1 is a is a medium-altitude long-endurance (MALE) Remotely-Piloted Air System (RPAS). The RPAS air vehicle is monitored and controlled by an aircrew in a ground control station, who are also responsible for employment of the weapons.  Operation of the RPAS is achieved through waypoint control, with the air vehicle flying autonomously between waypoints (ie no direct hands-on flight control capability); landing and take-off are achieved autonomously.

Design.  The Ghost RG Mk 1 platform has a blended wing body design with an inverted V-tail structure. Thrust is generated by a variable pitch two-blade propeller, in pusher configuration, mounted between the tail booms and driven by a gas turbine located in the body. Fuselage pieces are made mostly of carbon-fibre composite with machined aluminium parts at joints. Fuel is stored within bladder tanks and fuel consumption is balanced with solenoid valves.

Certification.  As a Military Off the Shelf (MOTS) derivative, the Ghost RG Mk 1 is designed to NATO STANAG 4761 – Uncrewed Aerial Vehicles System Airworthiness Requirements (USAR); STANAG 4671 is a key component of Def Stan 00-970 Part 9 – RPAS.  However, during UK certification of this RPAS, it was established that the Ghost RG Mk 1 was non-compliant with Def Stan 00-970 Part 9 because does not have a pitot-static system anti-icing capability.

USAR 1323      Airspeed measuring device

If a pitot tube is used then the following applies:

(a) The design and installation of each airspeed measuring device must provide positive drainage of moisture from the pitot static plumbing.

(b) Each airspeed measuring device must have a heated pitot tube or an equivalent means of preventing malfunction due to icing.

Although this non-compliance was not a problem in the hot and dry country of design (and was therefore accepted by the original certifying authority), this non-compliance has, unfortunately, resulted in a number of safety of flight incidents in the UK, including one accident, due to the colder / wetter environment.

Accident.  The Service Inquiry into the accident concluded the cause of the crash was aerodynamic stall. The Ghost RPAS stalled because the Flight Control Software (FCS), within the Flight System Control Computer (FSCC), was using incorrect air speed values. These values were calculated by the FSCC from pitot total pressure measurements but were wrong because pitot tube had become blocked following prolonged periods of flying in cloud, precipitation and icing conditions. The FCS’s response to these erroneous air speed inputs caused a loss of control at 120 kph resulting in severe pitching oscillations, over-speeding and unstable flight near the aircraft’s stall speed over a period of 90 seconds. The Ghost RPAS finally stalled and spun from 4,500 feet, crashing into a small woodland, 2 km along and 1 km left of track, from of when unstable flight commenced.

Training.  Currently undertaken from an MOD airbase on the Moray Firth, Ghost RG Mk 1 training is planned to relocate to MOD Lyneham in Wiltshire, to provide better access to the Salisbury military training areas.  However, the direct route to and from these training areas (blue dashed line) overflies a number of congested areas (areas of population) and several busy main roads. 

 

Thus, the risk of a loss of control over these areas, and potential Risk to Life for third parties along and adjacent to the flight path, was neither deemed to be ALARP nor Tolerable by the Aviation Duty Holder (ADH).

Consequently, the Ghost Force Head Quarters (FHQ) have flight planned an ADH-approved ‘safe corridor’ (red solid line) which ensures that the Ghost air vehicle:

  • Crosses as few main roads as possible, and then at a tangent (shortest crossing).
  • Does not directly overly populated areas (defined as large villages / small towns) and flies no closer than 2 km adjacent to any populated areas.
  • Transits the safe corridor at 9,000 ft and 150 kph.
  • Avoids rain and known icing conditions.

Therefore, noting that there was no opportunity to design out the non-compliance, the TAA agreed with the ADH that the proposed ‘safe corridor’ established sufficient operating controls, factors and mitigations that would enable an Equivalent Level of Safety to be claimed.

MCRI

The MCRI can be accessed by using the following link.

The Briefing slides can be accessed by using the following link.

Further information for the MAA

As a result of the TAA’s ELoS proposal, the MAA met with the Defence Accident Investigation Board (AAIB) and Release to Service Authority (RTS) to review the Ghost RG Mk 1 accident to ascertain the validity of the TAA / ADH claims.

Noting that the Ghost RG Mk 1 accident occurred from a lower altitude and lower airspeed than is being proposed for the safe-corridor transit, both the along-track and across-track distances would increase.  Furthermore, whilst there was negligible wind prior to the accident, the Ghost RG Mk 1’s 40 kt windspeed limit (as stated in the RTS) would cause an increased along-track crash distance (if the wind were aft) or an increased across-track distance (if the wind were directly port or starboard to the RPAS).

Airspeed kph Wind Speed kph Height ft Along Track (km Worst Case) Across Track (km Worst Case) Notes
120 Nil 4,500 2 1 Accident conditions
120 Nil 9,000 4 2  
150 Nil 9,000 5 2.25 Proposed transit spec
150 75 (aft) 9,000 7.5 2.25 Ghost RTS windspeed limit is 75 kph (40 kts)
150 75 (across) 9,000 5 4.75 Ghost RTS windspeed limit is 75 kph (40 kts)