When climbing in IAS (FLC) mode in the 850, is there a way to view the selected airspeed? I “cheat” and have it displayed on my Streamdeck autopilot screen, but I can’t find a cockpit output of this number.
I don’t have much experience with this aircraft as I’m only trying to learn how it works, but from what I’ve understood so far you should be able to change your IAS speed with the UP/DN buttons of the autopilot. Here’s how these buttons are programmed by the dev himself (ref. that “AP_SPD_VAR_INC” event):
(A:AUTOPILOT FLIGHT LEVEL CHANGE, Bool) if{
(>K:AP_SPD_VAR_INC)
}
els{
(A:AUTOPILOT VERTICAL HOLD, bool) if{
(>K:AP_VS_VAR_INC)
}
els{
(A:AUTOPILOT ALTITUDE LOCK VAR, FEET) 33600 < if{
500 (>K:AP_ALT_VAR_INC)
}
However I’m not sure if the aircraft is operated like that in real life. From what I understand once the climbing starts, the pilot activates 850 Mode (that button above the flaps lever) and the aircraft will climb automagically keeping its optimal speed of 130KIAS, which at lower altitudes translates to about 2000 ft/min depending on throttle settings.
This is also how it works in the sim from my limited experience. So no need to mess with the climbing speed. Just enable IAS in the autopilot, also enable 850 Mode and you’ll start climbing at 130KIAS every single time.
Obviously I’m not saying the above is 100% accurate, as I’m still trying to figure this out. I’m not even sure 130KIAS is the speed to maintain all the way to your cruising altitude. I’m now reading a real-life account, that:
At 85, a tug on the yoke sent the 850 skyward and we settled into a 2,000-fpm climb doing 130 KIAS. When we reached 2,000 feet or so, the flap switch was moved out of the UP detent, and into the 850 position. Now we had to be careful about adding power in the climb. Push the thrust lever forward too much, and torque could break limits. Out came the engine operation charts.
All turboprops have two engine limitations. At lower altitudes, torque is the limiting factor. But as torque inevitably falls off with altitude, ITTs climb in the thinner air. So up high, ITT becomes the chief limitation. For the 850’s PT6A-66D, however, that huge 975-shaft-horsepower difference between its thermodynamic rating and its flat rating gives you comfortable margins against over-torquing down low and over-temping up high. If you do manage to over-torque the engine, you have 20 seconds to pull the power and correct the situation before informing maintenance. With the TBM 700, that grace period is a mere five seconds.
Passing through 17,000 feet and climbing at 160 KIAS, we still registered a 1,200-fpm climb. And by FL190 the climb rate was the same, and our ITT was a comfortable 781 degrees Celsius (redline is 840 degrees). By FL220, our torque limits had dropped to 110 foot-pounds, but the ITT stayed at 781.
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Interesting, so it’s almost like it’s “soft-coded” to 130 KIAS, but you can override it? I’ll have to play with it more, though I’ve been using 130 as my climb speed anyways. I use Spad.Next and could hard-code it to always climb at 130, but as you mention, the up/down keys on the AP panel can (theoretically?) modify the speed…I’ll have to try that and see if my Streamdeck IAS display changes.
The way IAS mode is supposed to work is that it will hold the indicated airspeed you are currently flying when you press the button (provided AP is engaged). It does this by controlling the aircraft’s pitch to manage the resulting energy state. Always keep in mind pitch + power = performance. Also, understand we climb due to using power or thrust greater than what is required to maintain level flight at a given angle of attack, descend due to the opposite.
The key in IAS mode is to either set the power to achieve a specific climb or descent rate at the selected speed, or just use a fixed power setting and accept whatever climb rate you get. Notice this is NOT the same as VNAV with autothrottle, in which the throttle is also controlled to get both the intended climb rate and indicated speed (or Mach, whatever).
So, for example, if you’re trimmed for level flight at 200 knots and engage IAS mode, it’s going to hold 200 knots and may be mostly level, but if you increase the throttle, it has to do something with that excess power that would normally result in an increase in airspeed. So to hold the airspeed, it will pitch up and enter a climb, maintaining that 200 knots. Conversely, if you were level and reduced the throttle, it will pitch down to maintain that 200 knots.
For a climb, set the throttle for max continuous climb power (or using whatever other limitations like ITT), then pitch for 140 or 160 knots indicated, engage IAS mode, and accept whatever climb rate results. Power setting, weight, density altitude, and configuration will all change the climb rate at a fixed airspeed. If you reach the limit of a rate of climb at a particular airspeed (like absolute ceiling at Vx/Vy), and power setting, while in IAS mode, it will no longer be able to maintain a climb.
Pressing the up and down buttons while in IAS mode should respectively increase or decrease the selected IAS by 0.75 knots/second while the UP or DN button is held. For example, if you’re climbing at 140 knots in IAS mode and select UP to increase speed, without changing power, it will have to lower the pitch to accelerate and you will (at least momentarily) get a lower climb rate.
After all of this, if you prefer to climb at a faster or slower rate, you need to manage the power setting or just set vertical speed mode and accept whatever IAS you get for the combination of that and your power setting.
That said, I have no idea if the sim implements the correct logic to all of this. However, I was flying the TBM850 a bit yesterday to test some of my button box bindings and it was holding IAS like a champ at whichever speed I was flying when I pressed the button.
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I’m not a pilot but I’m not seeing any automation in 850 mode, just the ability to set 120 torque and dial in a decent climb rate
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850 mode just removes the torque limiter that normally limits torque to 110% when not in 850 mode. But what is probably being observed is, say, you’re in a climb at a steady airspeed and engage 850 mode, you may suddenly have a bunch of extra power/thrust, so your climb rate will increase.
When engaging 850, prop RPM should be 2000 and torque should be less than 100%.
Once 850 mode is engaged, be careful not to exceed the red line at 121.4% TRQ
When coming out of 850, you should be at or above 1500 AGL, at 2000 RPM, and power below 100% torque before disengaging.
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Hmm, yes. By a very quick look at the code in “Analog TBM.xml” it doesn’t seem there’s any automation involved. Just that the max torque limit is updated.
So as CharlieFox00 said, I understand the idea is that initially you’re flying as a TBM700:
Let’s say you carelessly ram the power lever forward during takeoff, with the flaps set in the TO position. When the torque limiter senses that you’ve reached torque limits (100 percent on the torque gauge), fuel flow is reduced, torque is kept under control, and horsepower is limited to 700 shaft horsepower. For this reason, you’re basically flying a TBM 700 every time you take off. And this is normal procedure.
Once you’ve fully retracted flaps after takeoff, when passing 1500AGL you first make sure you’re at 2000 RPM and thrust lever is reduced a bit so that torque is slightly less than 100%. Then you move the flaps detent to 850 Mode. This causes the engine protection to be removed completely and the torque limit increases to 121.4% instead of the original limit (which I assume is 100% but I also saw 110% at sea level in https://pohperformance.com/TBM/index.html, perhaps that’s only for limited time like 20 seconds).
Then what happens is:
As you climb, torque limits must be consulted by checking tables in the performance section of the 850’s pilot’s operating handbook. The torque limits drop with altitude, so you must fly by the book. Simply pushing the thrust lever up to the top of the torquemeter’s green arc as you climb can over-torque the engine and cause serious damage. It’s the price you pay for having all that extra power.
I haven’t full gone through the manual, but it doesn’t seem to include these performance tables (there’s some table called “Climb Performance 130 KTS - Standard Day (ISA)” but I don’t think it’s what we’re after). I took a quick look at the actual Daher POH and it seems to have climbing scenarios for 130 and 160 KIAS, which seem to be the 2 optimal speeds for climbing. There it also shows how torque limits are dependent on bleed operation (which suggests that for HI bleed, max torque should be limited to ~115% or so).
I think that POH performance tables later suggest that both for climbing and cruising, a torque of 121% is to be maintained until 25000 feet or so, after which we gradually decrease to 98% at the service ceiling of 31000 feet (I assume that’s because ITT becomes the more important factor at these altitudes).
Then we keep 850 mode on during the entire flight and only go back to 700HP when we’re about to land:
The procedure on the prelanding checklist is to return the flap switch to the UP position by 1,500 feet agl or so. Now you’ve got torque limiting, and a return to a maximum of 700 shaft horsepower. The idea is to have torque limiting online in case of a go-around or missed approach. Takeoffs with the flap switch in the 850 position are prohibited.
And these only seem to be the tip of the iceberg.
Oh my, I’m coming from C152/172, CJ4 and CRJ where engine and performance management was so much easier. This seems overwhelming at times but it’s great fun.
A couple things:
-
The POH you linked is for a later 850 model with a G1000. There are some slight differences with the analog version, weights differ, systems, etc.
-
Recommended best climb speeds may depend on weight.
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Climbs in TBMs are book-spec’d to max climb power (limited to 121% TRQ) or 700 shaft horsepower (SHP), limited to 100% TRQ. Use the engine performance climb table to find the max torque for your climb profile, altitude and OAT, then add 1% TRQ (within limitations) for every 10 knots over 130 knots. Bleed reductions come in after all that.
-
Don’t exceed ITT or NG limitations (amongst others), the margins for which are constantly changing based on time at a power setting, altitude, airspeed, and/or temperature.
-
This is all based on the book - actual operators may key on one value over another or might be more conservative to minimize engine wear.
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Amazing. I guess the keyword here is being “conservative” (especially when unsure) rather than try to squeeze every drop of performance from the aircraft.
Hopefully this is the one? (source, page 5.8.2)
Is this coming to Xbox ?
That’s one of them. There should be four pages for climbs: Max climb power (MXCL) and 700 SHP climbs, at or below FL200 and at or above FL 200 for each.
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It seems the TBM is like the PC-12 in that they both don’t display the IAS set value. Most others do, so your expectation to want to see that info was valid. I personally find it a bit strange that they both chose not to when they have the kit on board to do so. Yes it will capture the current speed when first engaged but if you provide up/dn buttons to adjust it after that then you should really have the value indicated somewhere like many other aircraft do.
If the real aircraft is like that perhaps it is intentional. After all with no auto-throttle the IAS selected is more aspirational than anything else.
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It is indicated, on the airspeed indicator.
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Either you don’t understand what’s being discussed or that’s just a throw away comment. Either way it’s wrong.
I understand exactly what’s being discussed. A separate IAS indicator would work well in the case of an integrated system that includes an autothrottle, but is totally unnecessary for a system that only controls pitch to maintain the indicated airspeed at which the airplane is flying when the system is engaged.
The airspeed on the dial is literally the IAS speed. I don’t need a second indicator to tell me what it’s set at, even if I make changes. The only time it won’t indicate what it’s set at is when you’ve exceeded limitations and it can no longer maintain that. If you get to that point, you have other issues.
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This is not how it is supposed to work. If people would just watch some steveo1kinevo YouTube movies, all would be clear. The BS TBM850 flies very similar.
You just press IAS mode when you are at a desired airspeed. If you want to change that speed, leave IAS mode for whatever you want (AP off or VS mode), get to your new desired airspeed and then press IAS again.
There is certainly no automation that automatically sets speed to 130 kts during climb. Using the 850mode is entirely voluntary, but off course recommended if you want to climb faster or go faster!
The IAS indicator is not the target is it now and the point being many other non auto-throttle aircraft in the sim have the target displayed. Not suggesting the TBM is wrong just strange that’s it and the PC-12 are different from most of the others. That’s all.
That may be how he uses it but unless the up/dn buttons are disabled for the TBM then its possible to adjust the target using those controls provided and with that strange that no display of the target is provided. Just exploring why when we always bug and display other targets such as HDG, ALT and VS why we don’t always for IAS. These are the first couple of aircraft I’ve come across that don’t have it if the AP supports it.