Flight Model used on Big Sims

[Pugachev Cobra]

Hello folks,

Does anyone know or has clues as what are the flight models used on Boeing/Airbus/etc training simulators?

Or perhaps which mathematical approach they use, like blade element theory?

Perhaps it's proprietary, but maybe someone here has some clues.

If they use it to recreate, e.g. upset accidents, and how the crew reacts, then obviously it's important that the flight model is as accurate as possible.

[Mad (Flt) Scientist]

Flight models on training sims, and used by manufacturers for engineering analysis, are inevitable data-based and in effect empirical models. Things like CFD are not used in the model (though may be used to extend the dataset to areas of the flight envelope where no other data exists).

The most reliable part of the model is the part covered by flight test data, where a pretty standard set of dynamic/kinematics equations is allied to a fairly standard set of aerodynamic coefficients and derivatives (CL-alpha, Cn-rudder, and so on) all defined as tables of data points as a function of the significant variables (so CL may be defined as a multi-linear function of alpha, Mach, flap position, etc.).

The data are derived from analysis of the data collected in flight test, and then confirmed by "matching" the model output to flight test output with the same set of inputs. (Pilot put in 3 deg elevator, aircraft pitched 2 deg/sec, sim model pitches 1.9 deg/sec, close enough, next data point ...)

Where no flight test data are available wind tunnel data may be used to extrapolate or interpolate from the flight data. Alternatively data for a similar aircraft may be used to guide extrapolation trends.

Only at the very start of a programme might CFD-based data be used, but even then the tendency is to start from a known model of a similar configuration and apply deltas, rather than start from scratch.

The approach of attempting to solve the flow conditions in real time is not used in industry. (Sweeping statement, but I've never heard of anyone tying that; it makes no real sense compared to doing the analysis offline, and would be near-impossible to obtain a sim qualification too)

[AnthonyGA]

Flight models on training sims, and used by manufacturers for engineering analysis, are inevitable data-based and in effect empirical models. Things like CFD are not used in the model (though may be used to extend the dataset to areas of the flight envelope where no other data exists).

How much of the flight envelope is usually covered by hard test data, and how much has to be simulated through extrapolation?

I've heard about simulators sometimes not being very good at extrapolating to flight regimes that weren't part of the tests. Wasn't there an accident involving extreme use of rudder controls or something some years ago that was traced in part to a difference between the way the sim behaved and the way the actual aircraft behaved.

[Mad (Flt) Scientist]

How much of the flight envelope is usually covered by hard test data, and how much has to be simulated through extrapolation?

I've heard about simulators sometimes not being very good at extrapolating to flight regimes that weren't part of the tests. Wasn't there an accident involving extreme use of rudder controls or something some years ago that was traced in part to a difference between the way the sim behaved and the way the actual aircraft behaved.

The minimal requirements for test data validation for a simulator are relatively sparse; FAA Advisory Circular AC120-40B "Airplane Simulator Qualification" (http://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentID/22762) is one example. (The regulations are pretty standard these days).

However, in practice there's a lot more than the bare minimum done. The simulator has to pass qualitative as well as quantitative tests, so the data has to be reasonable across the entire envelope, not just the specifics in the regs. And in practice many training simulator flight models start out as engineering simulation models in the respective OEMs, being used for design studies and for certification analysis, and so subject to further validation and oversight by the authorities. (A good example would be the use of simulation models for VMC extrapolation, or to provide loads data for extreme high speed manoeuvres; in both cases the certifying authority will expect a certain amount of validation specific to the conditions).

Similar considerations apply to systems simulation ; where systems are simulated (rather than "stimulated", which is when actuakl a/c hardware is used) the system models will trace their development back to various systems rigs and design tools.

The bottom line is that if you do something with a fully qualified simulator which is something previously done during the development or certification testing, or which is pretty similar, then the sim should be pretty well representative. Go outside those parameters, though - whether by exceeding the tested flight envelope or introducing combinations of failures never considered in design - and the sim becomes progressively less reliable as a guide to actual aircraft behaviour.

The case you are alluding to in the second para is I believe the use of the simulators in the AA upset training, which was discussed as part of the AA587 accident investigation. IIRC, one issue was that, in order to train the upset conditions, the sim was programmed - arbitrarily, IIRC - to be unresponsive to rudder inputs in certain flight conditions. One conclusion was that this may inadvertently have led the pilot to believe that the rudder was not just ineffective, but also benign.