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Low speed buffet quick rule of thumb

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Low speed buffet quick rule of thumb

Old 20th Jul 2015, 05:43
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Low speed buffet quick rule of thumb

I'm flying an EFIS 737 wo speed tape. How does one calculate the low speed buffet onset speed Up at altitude? (Assuming calm conditions)

I have heard some pilots use the vref40 speed plus a fudge factor... Is that a safe estimate? and if so, what should that fudge factor be?

Thanks in advance.

Last edited by Trash8mofo; 20th Jul 2015 at 07:34.
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Old 20th Jul 2015, 07:06
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Why do you want to know? Stick to the flap speed schedule and within the recommended bank angle, and you will be well clear of the buffet.
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Old 20th Jul 2015, 07:40
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Because the flap manuvering speed is only applicable to 20,000 ft. What I would like to know is my coffin corner margin without the benefit of a speed tape. I haven't seen a buffet boundary charts in the AFM and I'm thinking some of the old hands here might have some advice.
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Old 20th Jul 2015, 12:22
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"What I would like to know is my coffin corner margin without the benefit of a speed tape."

If I may make a rash assumption that you do not intend to fly at an altitude greater than the FMC predicted 'Max', then you will be NOWHERE near CC and do not need any 'rule of thumb'.

Of course, if you seek to soar with the angels near 'coffin corner', then you need to consider high speed buffet as well.
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Old 20th Jul 2015, 13:07
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The variation of the buffet boundary with altitude is caused by compressibility effects, expressed by the Mach number. Those effects are too complex to fit any 'rule of thumb'. What you really need is the buffet boundary determined in the manufacturer's flight tests, and that should be in the AFM. If you can't find it in the AFM, why don't you ask Boeing's Operations Support?
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Old 21st Jul 2015, 22:29
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Why not give the pax the flight/fright of their life and just go and horse around a bit,see how it feels at `x+` degrees of bank/v Mach; then you`ll know...
or perhaps it might be worthwile doing it in the Sim first.,...
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Old 21st Jul 2015, 23:25
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I can't answer you question directly regarding low speed buffet at altitude so apologies for that.

If you have an FMC with the hold page function then you could use that to give you a sensible minimum speed to fly in the cruise at high altitude. Select PPOS but don't execute. Note the speed it generates and don't fly below that, thus remaining on the correct side of the drag curve.

If you don't have an FMC then use the FCOM tables for holding. They will tell you amongst other things a speed to fly based on weight and altitude. Again a safe and sensible minimum speed.

Good luck and let us know if you get anything firm.
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Old 22nd Jul 2015, 04:41
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Thanks khoasai. After having flown the NGs its hard to try to get as much protection/information from the EFIS 1.0 airplanes.
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Old 25th Jul 2015, 10:04
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Buffet is min speed but...

It is nice to know your low speed buffet for a particular weight and altitude, however I personally don't think that it is not too smart to be flying below drift down speed one eng inop.

I fly the 767 and we use as a ball park Ref30 + 100. (It is actual somewhere between +98 and +95, close enough). It would be easy enough to extract the figures for the 737 from the books.

If you know your min speed at sea level, you can simply add 1 knot for every 1000' above 20 000'. That equates the allowance for compressibility. This is important because aircraft stall relative to equivalent airspeed and not CAS.

Remember also that high speed buffet is in relation to Mach number but low speed buffet although it may be expressed as a Mach number actually happens at a EAS.

Good luck with the books!

Last edited by Colin Oskopi; 26th Jul 2015 at 06:00.
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Old 25th Jul 2015, 17:28
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00'. That equates the allowance for compressibility. This is important because aircraft stall relative to equivalent airspeed and not CAS.
Remember also that high speed buffet is in relation to Mach number but low speed buffet although it may be expressed as a Mach number actually happens at a EAS.
There is more to it than that. At high angle of attack (i.e. close to buffet/stall) Mach/compressibility effects start as low as M=0.3.

Therefore the low-speed buffet boundary at altitude is not constant EAS.

Last edited by Gysbreght; 25th Jul 2015 at 17:42.
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Old 26th Jul 2015, 06:20
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Min speed and drift down VS Mach no.

That may be so, however what we are discussing here is min speed at altitude. The figures are readily available in the book and they are quoted in CAS, as are drift-down speeds. Both are derived from stall speed adjusted for compressibility.

Here is how they derive it on the A330-300.

Min clean = 191kts at 140,000kg below 20,000'
Add 6 kts per 10,000kg above 140,000kg
Add 1 kt per 1000' above 20,000'.

Say 200,000 kg flying at 35,000' min clean would be 191+36+15=242kts

Interestingly, that is how the Airbus FMS also calculates it, and this speed is also the drift down speed one engine out. You will notice that at no time is Mach number considered.
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Old 26th Jul 2015, 07:03
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Colin Oskopi,

I believe the subject of the thread is "low speed buffet". Have you looked at the buffet boundary and the stall speed on pages 4 and 7, respectively, of the A330 FCOM 3.01.20?

Last edited by Gysbreght; 26th Jul 2015 at 09:12.
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Old 26th Jul 2015, 12:26
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Thanks for the input, but wouldn't the plane have a lower AOA during drift down and therefore is afforded more stall protection? Therefore min speed in level flight in altitude would need a higher speed than drift down?
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Old 28th Jul 2015, 01:43
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Min speed clarification.

The reason I don't fly below drift down speed is, 1. It gives adequate margin over the stall, and 2. Drift down speed is your initial target speed or min speed if you like should an engine failure occur. If you fly below drift down speed, and you do lose an engine you will have to descend immediately, and part of the procedure regarding RVSM is to minimise your altitude loss until 10 nm off track. Remember you will be turning as well. Do you want to be doing this at low speed?

The point is not so much what speed you chose as your minimum, but being able to justify why you chose that speed.

Back to the 767, Min speed at sea level is Ref30+80.
At 37000' +17 kts, min speed is Ref30+97, which is within a couple of knots of the book figures for drift down speed. There is method to their madness.

At no time does the book figures or the FMS give a Mach number.
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