Recently we have been working on a project where the production of the VDR was a major subject. For a lot of people they will only come across a VDR on a new FSTD project and even then the Training Device Manufacturer’s (TDM) team will take the lead. However it does still remain the responsibility of the operator that any MQTG VDR is correct and submitted to their authority with the QTG so we thought that this month we would take a look at VDRs, how they came about and what they are for.
A bit of history
We first encountered a VDR in the early 1990s during the B737NG simulation industry working groups and some A320 initial qualifications. At that time Boeing and Airbus supported the industry by providing data packages for all aircraft variants as dictated by the actual aircraft tail number being simulated, covering different engine and avionic fits amongst other variables. At the beginning of the B737NG programme three aircraft versions were planned with varying fuselage lengths, the -600, -700 and -800. All three variants were able to be specified with one of three engine thrust ratings; that would have meant there having to be nine sets of validation data gathering flight tests. And that was before the -900 version and the winglets options came about. A similar situation with Airbus meant we had two engine fits (CFM/IAE) potentially for each A319/A320/A321 configuration to be simulated plus the continuing evolution of the Electronic Flight Control System (EFCS) standards. Common type rating issues aside, clearly this was madness.
The solution the Boeing simulator data group came up with was, at the time, very innovative. They first realised that some aircraft performance and handling characteristic tests were predominantly unaffected by engine thrust ratings or body length changes. So for those tests, it was only necessary to flight test one aircraft variant of fuselage length or engine thrust rating since that data also applied to the other variants. But that still left a large number of tests that required specific aircraft variant validation data to be provided.
The next intellectual leap was to recognise that for some of these tests, even where there were differences, these differences were well understood, from an aerodynamic performance and handling aspect, and it would be possible to model these and provide engineering test rig data in lieu of flight test data.
That left a much-reduced set of fuselage length/engine thrust related tests that had to be collected on the aircraft by flight test; engine out airworthiness tests for Vmca/Vmcg for example.
The next step was to persuade the qualifying aviation authorities and the industry participants; for them they could see a nightmare of producing QTGs from a mish-mash of sources that would be complex to manage. Think of an FFS with multiple fits and multiple engines. The solution was the VDR as proposed and discussed by many of the industry players at various working group meetings and RAeS conferences. As some of the SIM OPS partners well recall this was led by Boeing’s Ken Neville and Robert Curnutt at the time using the B737NG as a test case to validate which test did not require additional flight test data or could be confidently based on the use of predicted engineering simulation data.
Of course, at this stage Airbus were ploughing their own furrow and for the A319/A320/A321 defined a single training standard based on the A320 and defining differences training for the other models. Several meetings between the regulatory authorities and industry in Toulouse with Airbus resulted in what was called the Recommended QTG or RQTG that was effectively a VDR by another name and covered all the rationales for which data could be used for which Airbus airframe/engine/avionics combination. Looking at that time from 2024 it seems strange that then the idea of OEM aircraft simulation standards was controversial, given how well accepted the concept is now. It is very rare to see a FSTD built to a data pack for a specific tail number today in commercial transport devices, business jet FSTDs being an exception usually.
So what was/is the VDR?
In essence, it is a table that lists every single QTG check that requires aircraft validation data and against each test lists the data supplier’s recommended source of data to validate the model against for that test. Where that data has not come from flight testing of the exact variant of the aircraft being simulated (for example a different fuselage length, thrust rating, FBW standard or engineering test rig data) a rationale is provided that can be included in the QTG presented to the authorities.
Typically it includes the following components:
Test Identification: Each QTG test is clearly identified by ICAO/IATA number and description.
Data Source: For each test the exact source of the validation data is specified. Be that flight test data, wind tunnel data, engineering simulation data, or other approved sources.
Data Applicability: whether the data is directly applicable to the simulated aircraft variant or if it's from a similar variant.
Rationale: When data from a different aircraft variant or an alternative source is being used, a detailed rationale explaining why this data is valid for the simulator being qualified.
Data Limitations: Any known limitations or conditions of the validation data are documented.
What about today?
Within EASA for type-specific FFS and FTDs representing aircraft certified after January 2002 a VDR is mandatory and it is encouraged for earlier aircraft types. Appendix 2 to AMC1 FSTD(A).300 lays out the expectations for the VDR and the responsibilities of the data provider. In this AMC there is one very clear statement, often ignored, “A VDR should be submitted to the competent authority as early as possible in the planning stages for any FSTD planned for qualification to the standards …” As we have pointed out in earlier blogs our experience is that the national authorities are there to assist, the earlier a potential issue is brought to their attention the sooner a solution can be found.
The VDR is also the place where EASA expects rationales associated with subjective footprint tests to be documented, footprint tests being the last resort for qualification where no other validation data is available (see GM1 ORA.FSTD.200). If footprint tests are planned you really do need to talk to your authority (preferably before signing a purchase contract!).
Under FAA the requirements are very similar, to the extent the example VDR extract used by both documents (CS FSTD (A) and FAA Part 60) are identical. The emphasis being on the data supplier's responsibility to provide one and to recommend the best available validation data type for each test.
So how does the VDR differ from the Validation Source Data (VSD)?
The concept of VSD is laid out in EASA document CS-SIMD rev. 2, part of the aircraft manufacturers' obligation to provide Operational Support Data (OSD). For aircraft certified after CS-SIMD came into force the data provider, the aircraft manufacturer or Supplemental Type Certificate (STC) holder, is obliged to collect data that feeds into the VDR. GM1 SIMD.120 Terminology provides an excellent graphic, see below, showing the flow from the VSD to VDR to validation data to presented QTG and finally the Master QTG.
Reference: Certification Specifications and Guidance Material for Simulator Data (CS-SIMD) Issue 2, 7 February 2022, page 6. Annex to ED Decision 2022/003/R. European Union Aviation Safety Agency.
What about tomorrow?
As we have mentioned in previous blogs, there is a paradigm shift in FSTD qualification underway resulting from the work in EASA RMT.0196 updating the CS-FSTD for fixed and rotary wing FSTD qualification to using the FSTD Capability Signature (FCS) concept. The NPAs are being published and an implementation is expected in 2026 Q1. Significantly this new release of CS-FSTD will mandate the provision of a VDR as part of an Engineering Report for the qualification of any and all FSTDs, including those devices we refer to as FNPTs today. Hence why it is essential that industry understands the concept behind VDRs and why they can be so important in FSTD qualification.
How can SIM OPS help?
Apart from being able to recount the birth of the VDR at length, we can guide customers through the process of checking the VDRs they are presented and advise where problems might or might not arise. Particularly where data sources are novel we can make sure your interests are well represented.
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