Real Airplane Question: why 850 Mode not for T/O
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Hey guys, I am enjoying the 850 tremendously. I have a (hopefully not to stupid question) about the real plane or more precisely the real engine of the TBM 850: why is the "850 mode" only allowed in flight and not for T/O? What is happening inside that engine with the 850SHP which is not suitable for the takeoff phase? Any hints or links are greatly appreciated.
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@dosmarder good question! also puzzled with it.... this is what I've found, nice reading:
https://www.flyingmag.com/pilot-reports-turboprops-tbm-850-even-faster/ -
@enrique thanks for sharing ;-)
very nice to read ! -
@Enrique Thanks for sharing, a nice and interesting read indeed!
Still I think the article doesn't really answer my question. One paragraph (quote see below) suggests that there was no need for more Takeoff-Power but this seems strange to me...who doesn't want more power in hot & high conditions? The only reason I can deduce from what I've read is that the protection of the torque limiter is more valuable than 100 or so more horse power which in turn means they we're not willing to develop a torque limiter for 121% torque...
Quote from the article: "Since the design objective for the 850 was to increase high-altitude cruise speed, and everybody was pretty happy with the TBM’s takeoff and initial climb performance, the decision was made to keep takeoff and initial climb power at 700 shp. The 850 weighs the same as the 700, which can use runways of 3,000 feet, so the decision makes sense. The 850 also retains the 700’s torque limiting system during takeoff and normal airport maneuvering, which protects the engine and reduces pilot workload."
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@dosmarder thanks, yep that's exactly the part that caught me. I think it makes sense for the manufacture to simplify the models differences and the certifications and training required.
From my humble real-life C172 experience, even a modest diff of 160HP vs 180HP models can feel quite different, especially when torque and p-factor are at their fullest.
As I understand, the goal was to focus on the cruise performance without dealing with the rest of the flight envelope. While I'm also a bit puzzled about the lack of 850 mode during takeoff ( and go-arounds!) , I guess they considered the available performance for T/O GA balanced, proven and well in the design goals.
Another hint I've found, relies in the differences between the 850 and 900 where the 850 SHP made available for the entire flight regime. Here is an excerpt:
"The new dorsal fin and a new torque limiter on the PT6A allow takeoffs at the engine’s fully flat-rated 850 shp instead of the 700 shp limitation in the earlier TBMs. " https://www.ainonline.com/aviation-news/business-aviation/2014-07-01/pilot-report-daher-socata-tbm-900
My another hint also comes from the 850 POH, especially interesting to read is the restriction to use "850" during a go-around:GO--AROUND (2/2)
If IAS is at or above 115 KIAS :
8 -- Flaps ................................................ UP
In case of air leak between the solenoïd valve and the torque limiter,
the available torque might be below 100 %. Consequently, it is
strongly recommended not to select “850” :
for a new approach or visual circuit
for staying below 1500 ft AGLJust my guessing here, but it seems that the torque limiter is a sensitive point of failure and again in the balance of safety vs performance it is best to stay on 700SHP.. my interpretation anyways...