Overspeeding

Hi,

I’ve searched for answers but didn’t find any appropriate answer. Just this topic:

The thing is I know about the “speed limits” (below 10k ft. it’s maximum 250kn and above 0,90M, over 40k ft. no speed restrictions), but what I don’t know for sure is, that if I cruise typically at 33-37k ft. @0,82-0,83M IAS, I shouldn’t exceed 550kn…is that right? If yes, lately I’ve tested it with winds over 140kn coming from the back, so my ground speed was like 620kn, but didn’t receive any warning…
So my question is slow it down before I go to sleep on a long hauler or keep it for example at 0,83M and don’t care about the ground speed?

Ground speed is different from airspeed. An excess of airspeed is what gives you a violation.

More on that here:

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In some airliners such as the 787, normal cruise speed is M.87. But yes, you’ve got the right idea. Groundspeed in the “overspeeding” situation is irrelevant. Overspeeding is only relevant to the indicated airspeed. Overspeeding is depicted by the red dots on the airspeed tape.

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Thx guys for the explanations, understand the red dots and the IAS…that raises one more question: let’s say I will set my IAS to 340kn(and the red dots start at 350kn) with wind speed 50kn from the back…if the wind speed goes up to 150kn, will my IAS raise aswell? Or just the ground speed which, if I’m not mistaken, is irrelevant?

Btw. I fly the 787 maximum @0,85M…don’t like the idea of a violation and TS :)

If you have a tailwind or an increase in windspeed your IAS should not change by very much. Maybe ±5kts initially but your SPD hold should keep you at the speed you had set. Again, the ground speed is irrelevant. Overspeed only deals with the IAS.

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???

I thought the typical cruise for the 787-family was Mach 0.85? Have I done it wrong all this time… 😕

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M.85 is cruise, M.89 is max cruise

So what is the M0.87 you mentioned? When the cruise normally is .85 and not .87?

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I’ve found it to be an economy cruise speed. Its a bit closer to the sweet spot. When comparing .85 to .87, you get to your destination a bit quicker while using slightly less fuel in regards to the ratio of flight time to fuel used.

So the fuel consumption is ever-so-slightly lower by flying M.87? With other words, it is a little more efficient to fly at .87?

Yes. Let me see if I can simplify it. When comparing M.85 to M.87 you are:

Flying faster and you get to the destination in a shorter amount of time. Now, the nose of the aircraft is a bit more level to the horizon and fuel burn is slightly lower at this higher speed than a slower speed.

When at M.85, the opposite is true. Nose is slightly higher, which requires a slight amount more thrust (extra fuel burn). And it takes slightly longer because you’re flying slower. Again, the difference in minimal for a 3hr flight or shorter. But when flying from say LAX to SYD then you’ll see a difference. Hope that makes a little sense.

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Aha, that makes a lot of sense, thank you for taking your time to explain Deer. Very kind of you :)


Two more questions though, last ones, I swear 😅

  1. Do airlines usually fly at M.85 or .87 when flying these Long Hauls from for an example: US to AUS and/or Asia?

  2. Flying at .87, it that something pilots do when the flight is delayed and are perhaps trying to recover lost time?

  1. I’m not sure. It depends on what the dispatchers planned on. But I’d assume slightly faster. It all depends on if the aircraft is capable of flying those speeds.

  2. Sometimes. They may also check with ATC and/or company to see if pilots are giving ride reports. The pilots can talk with their company to find out where the better winds are at in terms of stronger tailwinds or headwinds that are not as strong.

Uh huh, that sounds reasonable. Thank you for answering my questions. I got to learn something new and very valuable today. I’ll be sure to research and check more into this now. Thanks :)

I’ve done most of the hard work already in regards to finding the economy speeds. I’ll just throw this here if you’re ever interested. I haven’t updated it in a while but it should give you a good idea.

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Just an aside, this is an old rule based on regions in IF. It no longer applies.

In Infinite Flight, the 787 flies slightly heavy, or has some other bug that allows the nose to rest higher than it would in real life, prompting the need for extra speed (or in my case an edited fuel bias on SimBrief). In talking to a 787 pilot for United on Instagram, I have learned that the most common cruise speed is .85, with .86 and .84 mixed in as well. For extra confirmation, SimBrief uses the real speeds of aircraft, and has never filed me faster than .86 (on occasion). .87 and .88 are viable speeds and may be used in some rarer cases where it makes sense economically, but the efficiency of the 787 on normal flights peaks at around .85.

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