ATC request Mach 0.83 to Mach 0.79
 

Hi,

What's your way of determining if the aircraft can meet the request instead of just winding the mach no. down and ensuring it stays above min. man speed?

Thanks

We have a chart for it, that I would look it up in.

Aircraft type?

https://i.imgur.com/madLs80.jpg

Aircraft type: B777

It was a while ago but I think my ballpark was 5kts per 0.01 mach ... and with each 0.01 Mach you'll lose 1 minute per hour on the ETAs .... instant calc and you don't have to touch the FMC etc

Thanks. Any reference material for the 5kts per 0.01M ?

a/t off, set speed bug to desired speed. Instant, and 100% accurate, presentation.

Rough estimate is 6 KIAS per .01 Mach.

As GBS says it’s ballpark stuff and FWIW it’s what I’ve always used....as for needing a “reference” or “reference material...” well TBH it’s a function of the basic school Physics and basic maths:

https://www.ck12.org/physics/speed-o...f-Sound-MS-PS/

As a clue well, Mach 1 at cruise alt temperatures is very roughly 10 nautical Miles a minute or 600 nm per hour..the rest follows....

Just bung the basics into a CRP computer .... or some of the great apps that are available and you'll be able to prove it to yourself. Some people might argue a knot or so either way but that's close enough for government work - especially at 2am 20 hours into a 30 hour day.

Alternatively, in the cruise disconnect the autothrust and see for yourself i.e click back the mach knob and look at the IAS change ........ if your airline allows you to touch the thrust levers :-)

The example you mention is also great when ATC says cross x at time Y. If the FIR etc is, say, 2 hours away and you need to lose, say, four minutes - can you reduce that much ? Well 5kts per 0.01 - does it keep you above minimum? then how much do you need to reduce without spending 10 minutes in the FMC? - Well 1 min per 0.01 per hour .... so just knock the speed back 0.02. Answer to ATC as he finishes the question !

There are also some great aviation "rule of thumb" sites out there with some real gems - all from the steam driven days of aviation but they still work.

My favourite was, the TAT at 18,000 feet equals the ISA deviation at top of climb ...... a real help in the old days for heavy widebodies (I also proved that one too from the equations but don't ask me to find it)

Cheers

Or you can just use the RTA function to meet
time over a waypoint requirement

...... if you have spent the time filling the FMC with all the data it needs to make the calculation ....... it's probably improved but I seem to remember it constantly changing its input and prediction. .... at least while it works it out you'll know the correct answer :-)

I think you're confusing it with TAS.

For IAS it's about 3kts per 0.01M.

You don't need a reference, it's just basic maths. If you are doing M0.80 and 240 KIAS, simply divide one by the other and get 3.

Likewise with TAS, if you are doing M0.80 and 460 KTAS, divide one by the other and get a number somewhere between 5 and 6 (closer to 6).

So if ATC want you to reduced your groundspeed by 30kts, then that's about M0.05, which will be about 15 KIAS.

My company's SOP (B73NG) is to use Vref40+100 as a minimum for enroute speeds. Works for us and no need to look up any charts.

Have a look at the hold page, select PPOS, don’t execute, note the IAS and convert to Mach number. Use that as a nice safe minimum speed to fly at in the higher cruise levels. The RTA function will give you virtually the same info but takes longer to calculate.

Thanks for this

Another rule of thumb. If recovering from a stall at (say) 37,000 ft, lower the nose to just below the horizon and keep it there until reaching Vref 40 plus 100 knots (737) or approx. 230-240 IAS then gently level out. Count on losing at least 3000 feet. This figure from the 737 FCTM for best holding speed above 25,000 ft if FMC not available. If stall was due to mishandling in severe turbulence, then increase to severe turbulence speed before attempting to level out.