Clarification about Penetration Speed on A320

[pineteam]

Hello Everyone,

It seems to be a grey area concerning the penetration speed amongs the pilots in the company i'm working for. Is it a structural or aerodymnamic limitation speed? I beleive it's Aerodynamic since the penetration speed increases from 250kt to 275kt when passing Fl200.
Some says in turbulence you have to fly at penetration speed: not slower, not faster! (eg 250kt below fl 200), some others will say you can fly slower but not faster than that speed and others will say it's just an aerodynamic speed target to stay between VLS and VMO and you can reduce or increase accordingly. I tend to agree with the last statement but I'm not 100% sure and I found unclear information in the QRH and AFM. The QRH says it's a " recommended speed" and in the AFM it says " respect the max penetration speed". Does it means if I fly at 300kt in severe turbulence at FL 120 we might damage the aircraft? I never read anywhere that flying faster than penetration speed may damage the aiframe. So I'm not sure I undestand the exact meaning of penetration speed by Airbus.

Thank you for those who can help me. 😁

[seen_the_box]

Does it means if I fly at 300kt in severe turbulence at FL 120 we might damage the aircraft? I never read anywhere that flying faster than penetration speed may damage the aiframe.

Airbus say that in case of severe turbulence, a tech log entry should be made, so that suggests that whatever speed you fly, there's a risk of airframe damage requiring maintenance action.

[Metro man]

Flying too slowly can stall the aircraft (cause the protections to activate) and flying too fast can over stress the airframe. The QRH OPS data section has a table of speeds and power setting for severe turbulence and recommends considering descent below optimum altitude to increase buffet margins.

[CaptainProp]

I believe the FCOM / FCTM recommends descending down to 4000' below optimum level to increase margins.

Interestingly enough I believe there is a difference in the speeds/thrust settings in tables in QRH and FCOM vs the graph in same section in FCOM. Don't have access to the books from iPad but I think graph is showing constant Severe Turbulence speed of 255(?) from FL200 (A319) but tables are showing .76 at FL330, 280 at FL310 and 255 at FL150.

[Togue]

pineteam,

Airbus recommends "aim to keep the speed in the region of the target speed, so as to provide the best protection against the effect of gust on the structural limits, whilst maintaining an adequate margin above VLS"(PRO-SUP-91-10 P 1/4).

The chart for Severe Turbulence on FCOM is identical to the one in the QRH.

CaptainProp is right. Severe Turbulence speed is 250kt from SL to FL200. Then is 280kt/M 0.76

Va ( Maneuvering Speed ) is a limitation. And it only applies when in alternate or direct law because in normal law you have protection. As per the graphic is from 256kt at SL ( is not linear ) to 300kt at FL295. Then 0.78M (FCOM LIM-13 P 2/6)

[CaptainMongo]

Our FM:

"Turbulent air penetration speeds provide an optimum margin between low and high speed buffets."

Interesting that our limitations are:

Severe Turbulence (Recommended)

Above 20,000 feet: .76 Mach/275 KIAS

Below 20,000 feet: 250 KIAS

When our company modifies Airbus FCOM wrt limitations they annotate it in the Limitations chapter (i.e. - company limitation) - in this case they did not, so I don't know where the speed discrepancy (280 vice 275) above 20,000 feet came from.

[FlyingStone]

There are some misconceptions with A320 (and possibly other Airbus FBW aircraft) about turbulence in cruise...

1. There is no special Airbus procedure, unless you are in severe turbulence. Below is an excerpt from FCTM regarding moderate turbulence:

If moderate turbulence is encountered, the flight crew should set the AP and A/THR to ON with managed speed.

2. If you have really severe turbulence, it's probably best to set fixed thrust and go along with the speed fluctuation rather than have A/THR go from idle to CL and vice versa to chase the desired speed.

If severe turbulence is encountered, the flight crew should keep the AP engaged. Thrust levers should be set to turbulence N1 (Refer to QRH), and the A/THR should then be disconnected. Use of the A/THR is, however, recommended during approach, in order to benefit from the GS mini.

3. A lot of stuff are valid only when you don't have protections (e.g. alternate and direct law). This includes, but is not limited to descending 4000ft below REC MAX FL (and not optimum), respecting VA, ... If you are so unlucky to get into alternate or even direct law in cruise, the ATC would probably want you to get out of RVSM at the first practical opportunity anyway.

[Escape Path]

Originally Posted by CaptainMongo

Our FM:

"Turbulent air penetration speeds provide an optimum margin between low and high speed buffets."

Interesting that our limitations are:

Severe Turbulence (Recommended)

Above 20,000 feet: .76 Mach/275 KIAS

Below 20,000 feet: 250 KIAS

When our company modifies Airbus FCOM wrt limitations they annotate it in the Limitations chapter (i.e. - company limitation) - in this case they did not, so I don't know where the speed discrepancy (280 vice 275) above 20,000 feet came from.

I have the same speeds as you have in our company's FCOM. Although mine quotes 275 up to FL310 and 0.76 above that

[Skornogr4phy]

There was an interesting article in a recent edition of Airbus flight safety. At high FL's overspeed is vastly preferable to underspend. The gap between MMO and the speed at which severe airframe damage is quite large. At low FL's the gap between VMO and airframe damage is much smaller. However it's easier to recover from a stall at low altitudes (not in normal law obviously) than it is at high altitudes.

[pineteam]

Thank you all for your inputs... Much appreciated.

[CaptainProp]

A lot of stuff are valid only when you don't have protections (e.g. alternate and direct law). This includes, but is not limited to descending 4000ft below REC MAX FL (and not optimum

Yes, but FCOM and QRH states "Consider descending to or below OPT FL in order to increase the margin to buffet" in several places and this for normal ops in normal law.

CP

[Bula]

To answer your question about faster/slower......

Vb is the fasted speed which you can fly without the aircraft exceeding its maximum structural load at the certified vertical gust.

Slower, and she will stall before over stressing, faster and she will only be able to stand a lower vertical gust prior to reaching to maximum design structural load.