* Clean speed?
 

Hello guys,

As far as I know, if ATC request to slow down to "clean speed", we have to decrease speed to Vref40 + 100 if we are above FL250.
And if we are below FL250, then we have to decrease as much as we see on PFD.

And the question is; why is it FL250? Any ideas?

Give us a clue. What aircraft type are we talking here.

Does it matters? Clean speed is clean speed for all types of planes, doesn't it?

The short answer is no, it's not necessarily the same. Is it a secret what airplane you're talking about?

ATC requesting clean speed above FL 250? That asks a question. Why on earth; or in the sky?

Maybe, but not Vref40+100

They ask you to pass a certain waypoint at time XX:XX, to avoid holding. The resulting IAS is often close to clean speed.

It's you specifying FL250 that means yes it does matter. If you're talking about a Boeing then that'll be a company defined FL and not a manufacturer. Airbus or any other manufacturer no I can't help you.

ok guys, I knew like it would not be different for plane type. It's not a secret ; I was primarily asking for B737. I2m pretty sure about Vref40+100 kts but I do not know why.

In the Performance manuals for the 744, holding speeds are increased by about 20 KIAS at 20,000' and above. I suspect it has something to do with stall margin at max angles of bank (since holding info includes all margins for the turns), but there is no explicit explanation.

The Boeing FCTM has a slightly more elaborate explanation, which addresses margin to buffet at full maneuver capability (as I indicated above). I suggest you check your FCTM.

Flaps up maneuver speed in the 737 is Vref40+70. Above FL250 Boeing recommends using Vref40+100 as an approximate min/holding speed to provide adequate margin to buffet.

Not sure if this is what you're referring to - I've only ever been asked to fly "minimum clean" at low altitude (in which case we just fly Vref40+70), if we're high they'll normally say "slow to minimum speed", in which case we maintain a minimum 10 knot margin above the amber on the speed tape.

Edit: I see you're specifically asking about the significance of FL250. I'd say this is a level Boeing has chosen above which flying Vref40+70 may not give adequate margin to buffet.

Manicured garden, I am assuming that the 737 is the only jet you have flown, because min clean speed is different for each machine, not the least because not all aeroplanes have a flap 40 to add whatever speed to!

The airbii and embraer jets have flap 1,2,3 etc, the 146/RJ has 18/24/36 from memory - not that it got over 25000 much!

so yes the aeroplane type matters

Suspect that FL250 is chosen as the altitude where difference between IAS and equivalent airspeed becomes significant in this particular type. I.e., above FL250, the F40+70 knots IAS no longer provides sufficient margin above the stall as the EAS has become too slow. On the 146/RJ the minimum clean speed increases by 20 knots above FL270. As alluded to by Snakecharmer however, this figure is largely of academic interest as the poor thing is more often found stooging around at ~FL250

It doesn't consider g-load. The red bricks will fluctuate with an increase in wing load but the amber bar won't.

They ask you to pass a certain waypoint at time XX:XX, to avoid holding. The resulting IAS is often close to clean speed.

Isn't it the case that ATC ask you what they would like you to do and you decide how to achieve it, or if it's even possible.

For the aircraft that I fly (Falcon900) I have been told that minimum maneuvering speed (protect you from stall in a 30 degrees turn) was Vref for your configuration (for this case Vref with no slats and flaps) + 20 Kt. That speed would be very close to Vfinal take off.
I haven't seen this written or explained in any book.

For me I feel that it is to low so I fly 200 knots when they asked me min. clean speed.

RAT5,

Isn't it the case that ATC ask you what they would like you to do and you decide how to achieve it, or if it's even possible.

You know that, I know that.

I was just giving a real example of when an ATC request has put me close to min maneuver speed in cruise. In fact, I don't find the RTA function particularly good anyway on the 737. It often starts with a very low cruise speed and progressively speeds up. Even with all cruise and descent winds loaded.

We have a green dot :-)

Don't forget that max alpha reduces with Mach so at higher alt you will need higher IAS.

This is simple; ATC are asking you to slow to your slowest, safest, zero flap speed. They are doing this to save you fuel and give them as much time as they can to make things work. ATC is trying their best to make the situation as good as they can for BOTH parties.

Your min clean being unsafe to fly, if I read you correctly?

I wrote that wrong.. I meant minimum clean speed of course