Matt Thurber, editor-in-chief at AIN Media Group, has been flying since 1975 and writing about aviation since 1978 and now has the best job in the world, running editorial operations for Aviation International News, Business Jet Traveler, and FutureFlight.aero. In addition to working as an A&amp;P mechanic on everything from Piper Cubs to turboprops, Matt taught flying at his father’s flight school in Plymouth, Mass., in the early 1980s, flew for an aircraft owner/pilot, and for two summer seasons hunted swordfish near the George’s Banks off the East Coast from a Piper Super Cub. An ATP certificated fixed-wing pilot and CFII and commercial helicopter pilot, Matt is type-rated in the Citation 500 and Gulfstream V/550. Based in the Pacific Northwest, Matt and his team cover the entire aviation scene including business aircraft, helicopters, avionics, safety, manufacturing, charter, fractionals, technology, air transport, advanced air mobility, defense, and other subjects of interest to AIN, BJT, and FutureFlight readers.

Editor-in-Chief

Matt Thurber

In early January, desktop simulation company HotStart released what is likely the most comprehensive and accurate business jet simulator—the Challenger 650—for the X-Plane 11 platform. The simulator took three years to develop and faithfully replicates not only the airplane’s complex systems and avionics but also real operational factors such as dealing with the FBO for fuel purchases, flight planning and FMS usage, sound such as the door closing, hydraulic pumps actuating, and other typical noises, and detailed failure modes.
“We finally have a desktop trainer of the fidelity where every button works,” said Keith Smith, founder of PilotEdge, a real-time air traffic controller service for desktop simulation. After having spent weeks learning the Challenger 650 simulator and practicing flying with it, he envisions its use for pre-type rating training or as a way to introduce non-jet pilots to business jet operations. A candidate planning to obtain a Bombardier Challenger 650 type rating, he suggested, could study the desktop simulation before going to the training event and have a much more comprehensive training experience.
For example, study windows and internal status displays show detailed schematics of the airplane’s systems, with depictions of valve movement, fluid flow, electrical output, and even communication over digital buses, which could help increase the student’s understanding before attending ground school. And practicing going from dark cockpit to engine start using the checklists and the proper steps could greatly reduce the amount of time the student needs to spend in a fixed-based training device or even the full-flight simulator (FFS).
Smith is so enamored of the quality of the HotStart simulator that he is trying to set up an experiment, to see if a student prepped in the Challenger 650 desktop simulator could climb into a 650 FFS and without any coaching go from dark cockpit through engine start in far less time than a typical type rating candidate.
Another opportunity would be to create a jet transition training course, to help future business jet and airline pilots gain some understanding of jet processes and operations before they get hired by an airline or go to type rating training. The Challenger 650 shares many systems and avionics features with Bombardier’s CRJ airliners and thus might be a suitable platform for that kind of preparation. One of the big challenges faced by airlines hiring new pilots is that many of the pilot applicants have no experience operating jets and often require extra FFS time to overcome their weak areas. Spending extra time with an accurately replicated desktop simulation model before starting the airline training process could reduce the attrition problem. And these kinds of preparation courses could be taught by remote instructors to help keep costs down.<img alt data-entity-type="file" data-entity-uuid="038bdeaa-0f6c-4183-835f-7655cfb4c334" src="/sites/default/files/outside-view-on-ground_.jpg" width="2100" height="1050" loading="lazy">
<h2>Why the Challenger 650?</h2>
HotStart co-founder and developer Saso Kiselkov (who goes by the moniker Totoritko on Discord, the online hangout for many desktop simulation fans), has long felt that the lack of good business jet simulations is a big problem in the desktop simulator world. There are replications of all sorts of aircraft, but so far only some light airplanes and airliners have been duplicated with relatively high levels of fidelity. “There's a big gap in between,” he said. “That’s a space I want to play around in.”
Kiselkov, a software developer, and his partner in HotStart released a fairly accurate simulation of the TBM 900 a few years ago. While not a full “study level” simulation, the TBM 900 was a significant development, a replication of a sophisticated general aviation airplane that gave users a more faithful experience. For example, users could go through all the checklist steps to start the TBM’s PT6 engine and deal with potential issues, such as a hot start, low battery, etc., that could cause expensive maintenance problems. The simulation also made users handle these problems, like having to recharge the battery or buy an engine overhaul if a hot start wasn’t handled correctly. The TBM’s avionics weren’t an exact replication of the real airplane’s Garmin G1000 suite, but it’s close enough to be a satisfying experience.
With the Challenger 650, Kiselkov wanted to try crafting a twin-engine business jet simulation, using an airplane with transatlantic capability, but not a large-cabin, ultra-long-range jet nor a light or very light jet. A key requirement was that he have access to the actual airplane and pilots with experience flying it. The Challenger 650 met all those requirements.
At first he thought a business jet would naturally be simpler than some of the airliners that others have replicated for desktop simulation. Many of these are so-called study-level simulations, so accurate that a simmer can learn nearly as much about flying them as a real pilot going through type rating training, except of course for the experience of flying a full-flight simulator and the airplane itself.
The Challenger 650 is much more complex than the ubiquitous Boeing 737, a popular airplane well replicated in the desktop and training simulation fields. The Challenger 650 has three hydraulic systems, “backups for backups,” three flight management computers, synthetic vision, a head-up display, and full required navigation performance (RNP) capability. “I accidentally picked the most complicated [airplane],” he recalled.
The avionics were also a consideration. While Kiselkov hasn’t announced plans for a next simulator build, he does believe that having replicated the Challenger 650’s Collins Pro Line 21 avionics, he could use those avionics building blocks in future simulator products.
<h2>Building a Simulation</h2>
Because Kiselkov did not have access to the aerodynamic flight model and all the data that make up a modern jet—which aircraft manufacturers provide to simulator makers—he had to figure out how to replicate the Challenger 650’s systems, avionics, flight characteristics, and failure modes. “[I’ve] been starting out with the precept that since I’m trying to simulate the real thing, it has to follow the laws of physics. If I can implement that, the real things should fall out,” he explained. “The decision to simulate nearly everything was because I knew I never would be able to intuit every possible behavior of the aircraft, engine, and systems state, but I can derive aircraft behavior from first principles.”
In designing the fundamental aspects of how the simulation works, Kiselkov could either use hard-coded tables of how various systems work, for example, if a throttle is set here, then the engine’s fan is running at such a speed. The same is true of roll rate versus the position of the yoke. And any data between the tabulated data can be interpolated. However, he said, “I do not have that level of access.” 
<figure style="margin:0;">
<div style="margin-bottom:5px;"><img alt="Study windows illustrate the level of detail behind the simulation of the Challenger 650." data-entity-type="file" data-entity-uuid="9bba4ef0-3dfa-4689-9ed1-44c7246a887a" src="/sites/default/files/engine-flow-2.jpg" width="2100" height="1050" loading="lazy"></div>
<figcaption style="font-size:1.5rem;line-height:1.2;font-style:italic;">Study windows illustrate the level of detail behind the simulation of the Challenger 650.</figcaption>
</figure>

So what Kiselkov did was “derive aircraft behavior from first principles. You imagine a hydraulic hose of a certain diameter and the fluid viscosity, and that should give me the behavior. It’s not hard-code tabulated. Then we do a little bit of tweaking here and there. I knew it’s the real thing; as long as I can get the physics right, it should come out.”
This meant a ton of work over the past three years; replicating the 650 was not a trivial task. “Doing the physics” meant locating many fine details such as dimensions and shapes of everything that makes up the airplane, engine pressure ratios, performance data, and much more. “I constructed the physical model and a simulator library into which I fed all the physical inputs. I started with data about the areas of the compressor stages, an estimate of what the turbines should do, the pressure and temperature drops. We’re mostly talking about turbomachinery terms, and compressor and turbine maps. Instead of getting engine data from [CF34 manufacturer] GE, I took some compressor maps that I found on the net and tweaked and tweaked till they matched the data.”
One of the advantages of this method, it turns out, is that Kiselkov was able to create a generic set of functions, like a simulator library, that accounts for the various engine parameters that are universal to a turbine engine. These might include drag coefficients of bearings, compression ratios, diameters of spools, etc. “This [library] is reasonably flexible,” he said, and could be used by other developers to mimic turbine engines of various types. In fact, Kiselkov used the library to replicate the Challenger 650’s auxiliary power unit.
Having access to a real Challenger 650 and experienced pilots made it easier to verify avionics, engine, and systems behaviors, flight characteristics like flaring during landing, and every aspect of flying the airplane. “The pilots recorded photos of various states of the aircraft,” he explained, “then I constructed test suites and ran the physics and compared and made sure we came up with the correct numbers, the known states for which we have data.
“The whole thing holds together physically,” he said. “There are no sleights of hand going on. For every condition, every system, I start with the fundamental physics, like conservation of energy. Whenever you want to move something it takes energy, and that gets propagated through the entire system. It’s all interconnected.”
For those who want to dive deep into the Challenger 650’s systems and understand how they work, the HotStart simulator includes in-depth study windows, with a myriad of details on each system. This is far more than anyone gets to see in a typical type rating course. An example of this level of detail is inertial reference system sensor performance, showing the “velocity random walk” in the x, y, and z axes. This is measured in “m/s/sqrt (HR)” and represents the error in white noise proportional to the square root of time.
Such deep detail is evident in every aspect of this simulator. The fuel computer, for another example, Kiselkov explained, “doesn’t get it right every time. It’s pretty normal during takeoff to see an increase in fuel onboard by 300 to 400 pounds. I’m not talking just the simulator but the real aircraft. The fuel computer is recreated to insane levels of detail; it’s like much of the rest of the avionics, it does not have any secret knowledge, it doesn’t know the exact value that’s in the tank, rather it just sees its simulated probes, and probes have noise and garble in them. The fuel computer has to filter and variously process that and correlate that to an internal table to what it thinks should be the fuel value. But plus or minus 4 or 5 percent on those fuel meters is perfectly acceptable.”
The HotStart Challenger 650 enables users to do en route controller-pilot datalink communications (CPDLC) as well as departure and oceanic clearances. This is basically a full implementation of the Challenger’s FANS capabilities, using a simulation of the Iridium satellite network for simulated satellite communications. 
<figure style="margin:0;">
<div style="margin-bottom:5px;"><img alt="Every knob, button, switch, circuit breaker, and control works exactly like the real airplane." data-entity-type="file" data-entity-uuid="f25b77b3-0b85-4b34-8072-4a9a2006025d" src="/sites/default/files/pilot-view-with-hud.jpg" width="2100" height="1050" loading="lazy"></div>
<figcaption style="font-size:1.5rem;line-height:1.2;font-style:italic;">Every knob, button, switch, circuit breaker, and control works exactly like the real airplane.</figcaption>
</figure>

<h2>Operating the Sim</h2>
When it comes to flying the HotStart Challenger 650, all the details come together to give the user an experience that simply hasn’t before been available in business airplane desktop simulations.
Word of warning: block off plenty of time to get to know the simulation; this isn’t an aircraft you can just fire up and fly away.
HotStart offers two ways to use the simulator, a training mode for quicker starts and a career mode that makes the user go through all the steps of getting a business jet underway. This includes placing a fuel order with the HotStart FBO, using the proper sequence of moves to open the cabin door, and removing the landing gear pins. Like the real airplane, the nose gear doors have to be opened to get to the nose gear pin. And you can’t open the nose doors without first turning on the APU and hydraulic pump 3A. If the engines need oil replenishment, then you have to use the automatic replenishment feature, which works just like the real airplane. Every system that generates audible sounds in real life is replicated in the simulation. “This level of fidelity is patently insane,” is the way PilotEdge's Keith Smith put it.
Once done with the external preflight, it’s time to climb into the left seat and go through the checklist. HotStart provides a regular checklist file that can be printed out and an expanded checklist that helps the user understand the steps involved. Once the avionics are on, you can use the built-in checklist in the Pro Line 21 avionics as well as an automated copilot who can run the checklist with you.
Having to go through all the steps just like the real airplane introduces the user to the location and functionality of all the switches, buttons, and knobs. Some of the checklist items are a little hard to accomplish, like the stick shaker and pusher test, using simulated controls, but there are ways around that, using programmable buttons on simulator flight controls. Smith adopted a generic midi controller’s blank buttons to program a lot of the Challenger 650’s switches, to save time going from place to place in the flight deck on his computer monitor. Setting up X-Plane “quick looks” for each key part of the flight deck does make navigating around easier, however.
Before taking off, not only do all the checklist items need to be completed, but the user must enter a flight plan into the FMS. For more realism, you can get a digital clearance using the built-in CPDLC or use Smith’s PilotEdge ATC service for CPDLC functionality. Creating the flight plan can be done using Navigraph’s SimBrief, which automatically sends the flight plan to the FMS. Even with the flight plan automatically loaded, the pilot still has to do all the normal FMS “hand jamming” to initialize the FMS, check and set performance information, move the flight plan to the active location, etc. If anything, this simulator can teach pilots who are unfamiliar with traditional FMSs how those complex but capable devices work.
The hard part of operating the HotStart Challenger 650 simulator is getting the airplane ready to fly. Once the user is good at doing that, the flying is the icing on the cake. There are plenty of YouTube videos available that show not only the startup process but also what it’s like to fly the HotStart Challenger 650 simulator, and it is highly recommended that new users spend some time getting familiar before making their own attempts.
Because the Challenger 650 is a two-pilot airplane, Kiselkov is planning to add support for multi-crew flying over the internet. This would allow two pilots with the simulator running on each of their computers to fly the same mission together. And this will open further opportunities for training new jet pilots on two-crew operations.

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