Risk of flying close to MMO

[extricate]

Hi there,

As per the below:

Overspeed Avoidance during Descent

Manual Flight at Crossover Altitude

When in descent close to MMO, if in manual flight (AP off), the risk of exceedance of the VMO at the crossover altitude is high. In this situation, the flight crew should know its crossover altitude and anticipate the switch to speed by reducing the aircraft pitch on approaching the crossover altitude.

Why is there a risk when MMO switches over to VMO if the pilot is flying at close to MMO? Can someone explain?

Thanks

[FullWings]

Because if you’re flying a constant Mach in descent, your IAS will be increasing as density increases. Your pitch attitude will be lower than that if you were flying a constant IAS, so if you are close/at Mmo at changeover, the tendency will still be to accelerate, possibly through Vmo if you’re not watching it carefully.

If you look at the flight envelope for a typical airliner, you will find a point around 28-29,000’ where Vmo and Mmo coincide. If you are climbing at a constant IAS, you will exceed Mmo eventually, in the same way the if you fly a constant Mach in the descent you will exceed Vmo on the way down.

[extricate]

Thanks for this.

[rudestuff]

At a constant Mach, in a descent the airspeed will be increasing. If you hold that Mach you will eventually reach Vmo (and wake the Captain up) - so when you reach an airspeed you're happy with for the descent, you pitch UP to maintain it. Now you're descending with a constant airspeed and decreasing Mach.

[Etropi]

May I suggest this reading for the topic, might contain some other useful info on flying close to MMO

High-altitude manual flying | Safety First (airbus.com)

:ok: (https://safetyfirst.airbus.com/high-altitude-manual-flying/)

[dixi188]

Is D.P. Davies book "Handling the Big Jets" still recommended reading?

[FlightDetent]

The manual quoted by the OP is poorly written, the message they're trying to get across is obfuscated. HtBJ does not help if the real question is "what are these guys talking about"?

[V_2]

It is not just manually flying where this is a threat. If descending using “FPA-Flight path angle” or “VS-Vertical Speed” modes to say make a level restriction, all can be fine when descending with a constant Mach number and there may even be a small amount of thrust still applied. But at transition the AP keeping that constant angle can cause the IAS to start running away and an over speed can easily happen if they crew are not effectively monitoring.

[Occy]

I believe, back in the day, if the overspeed warning wasn’t going off for most of the flight and you were flying.. beers were on you that evening.

[Private jet]

OP, Have a look at the "Chicken Tikka Massala" graph, in other words the relationship between Calibrated, True and Mach speeds vs Flight Level.

https://i.stack.imgur.com/BTKzz.png

[Check Airman]

Not sure what manual flight has to do with it. As has been mentioned, the same thing happens with the AP on.

[ScepticalOptomist]

dixi188

I think so - still some very useful information in it even for the new generation of jets.

[stilton]

rude stuff

Never had an issue on the rare occasions we were in that much of a hurry, rarely would ATC cooperate either

What’s being missed is VMO steadily increases as you descend, it’s not like you’re going to exceed it that easily

[hans brinker]

Private jet

not sure if this graph is correct. at fl350 doing .78/280 IAS the TAS would be around 450, not 350.

[Private jet]

Yes you're right, it's just one I quickly found online and the TAS scale is wrong now I look at it. I was just looking at the lay of the lines for the OP's original question. Getting "knowledge" from the internet is a dangerous pastime haha. The simpler version I was thinking of just has 3 lines, with any one speed constant the other 2 will diverge with increasing altitude and converge with decreasing altitude, thats all.

[hans brinker]

Occy

"operational check of the overspeed system. Ops check okay"

[hans brinker]

Private jet

BTDT! (been there, done that)

[Le Flaneur]

The reason is it so easy to exceed VMO when descending at MMO is the aircraft is accelerating more quickly than does at normal descent Mach. In order to maintain MMO, the pitch must be constantly decreased. If you wait until you reach VMO to transition to constant indicated airspeed, it is impossible not to overshoot. You need to lead with an increase in pitch about 10 knots prior to reaching VMO (probably varies by aircraft; this is my experience) . Most pilots will set off the overspeed clacker when flying emergency descents at MMO/VMO.

[vilas]

Most pilots will set off the overspeed clacker when flying emergency descents at MMO/VMO.
Well! Not in an Airbus.

[Capn Bloggs]

Stilton

You mean Vmax/the top foot increases...until you get to Vmo! :cool: