B737 Manoeuver speed

[TyRod]

Hello,

I was reading the fCTM the other day and noticed something about the minimum maneuver speed.

It says that below 20000ft or with flaps down, the amber band gives you 1.3g before stick shaker.
Above 20000, it gives you 1.3g before low speed buffet.

Can someone explain me why such difference have been made?

Thanks

[Avenger]

You have two completely different scenarios. As flap extension is limited to 20000 feet, the amber band with flap represents the minimum manoeuver speed which provides 25 degrees AOB plus 15 degrees overshoot before stick shaker logic activates. As stall speed varies with flap configuration it is better to view this as 1.3 VS approximately. Above 20000 feet the band represents 1.3g before low speed buffet, ( buffet alert in FMC display ) as the stall speed is calculated using gross weight, ambient conditions and AOB. If you are flying in LNAV you have "real time bank angle protection" so the aircraft will not over-bank, ( limited 8 to 15 degrees) but if you go to HDG SEL and leave 25 degrees bank you could see the amber bank creep. The new FMCs will show you as B-buffet limited or T- Thrust limited in the CRZ page. If you are flying B-limited altitude and the ISA deviation increases, you can also see the "buffet alert" message in the FMC as you may well now be above MAX ALT for the new ambient conditions. Manoeuvring in such conditions can be tricky.

[TyRod]

Hello,

That was not really my question or maybe I do not understand your answer :)

Why having chosen to set the maneuver speed from 2 different references:

stick shaker below 20 000ft
Low speed buffet above 20 000ft.

I know that below 20 000 ft, I have 1.3g available before stick shaker and in consequence some extra margin before reaching the stall speed.

Why did not we keep the stick shaker as a the reference for altitude higher than 20 000ft?

Do you mean that the stall speed below and above 20000 ft does not take the same parameters into acoount?

[Avenger]

Manoeuvre speed is not "set" by the stick shaker or the Buffet alert. It is a variable condition and flying at less than a certain speed for given weight and conditions will either activate the stick shaker or trigger the Buffett alert.

From the Boeing 737 FCTM: Quote:
"High Altitude Recovery. At higher altitudes above 20,000 feet, the airplane becomes increasingly thrust limited. If an approach to stall indication is experienced, nose down elevator and stabiliser trim is required to initiate a descent. This is because when the airplane is thrust limited, altitude needs to be traded for airspeed. Therefore a recovery at high altitude results in a greater altitude loss than a recovery at low altitudes".

In practical terms, at high altitude very heavy buffet will be experienced before a stall condition and recovery should be initiated before the activation of the stick shaker . At low altitudes the stick shaker often activates, there are bulletins from Boeing on this matter regarding VNAV and nuisance warnings with selection of anti ice etc etc, and recovery is much more straight forward.

Perhaps you need to look at the theory of energy management at high altitudes and low altitudes, it may help you understand the logic behind Boeings thinking.

Ref:

Stick shaker. An artificial stall warning system is required for airplane certification if the natural prestall buffet characteristics of the airplane are insufficient to warn the flight crew of an impending stall. This warning must be in a form other than visual to be effective, even if the flight crew is not looking at the instrument panel. Beginning with early commercial jetliners, standard practice has been to equip these airplanes with a stick shaker as a means of stall warning. Some airplanes also have employed stick nudgers or stick pushers to improve stall avoidance and stall characteristics. All these indications have been driven by an AOA threshold, which is usually a function of flap configuration, landing gear configuration, or both.

Because of the effect of Mach number on stall AOA, the stall warning AOA typically was set at a conservative level to accommodate gross weight and altitude variations expected in the terminal area.

Top of page
The early stall warning system thresholds were not set to be effective at cruise altitudes and speeds because they did not correct for Mach number (fig. 10). This kept the system simple. The stick shaker was set at an AOA effective for low altitudes but at too high a value for cruise. Natural stall buffet was found to give satisfactory warning at higher Mach numbers.
Later stall warning systems used Mach number from the pitot or static air data system to adjust the stall warning AOA threshold down as Mach number increased. This provided the flight crew with a stall warning related to the actual available performance. However, it also made the stall warning system dependent on good pitot and static data, a factor that will be considered in the next section on the dedicated AOA indicator.

It should be noted from figure 10 that the stall warning schedule does not follow the buffet boundary at very high Mach numbers. The buffet here is caused by Mach buffet, or too high a speed. Setting the stall warning system to activate at this point may lead the flight crew to believe the airplane is near stall and increase, rather than decrease, speed.

[TyRod]

Ok Ok I was not clear.

I was talking about the TOP OF THE AMBER BAR which represents 1.3g from stick shaker ( below 20000 ft) and low speed buffet ( above 20000 ft).

Why is the reference for the top of the amber bar different?

[Avenger]

Flaps UP or Flaps Down...

[eckhard]

It could be that with 1.3g applied above 20,000ft, low-speed buffet may occur before the stick shaker whereas below 20,000ft the stick shaker may activate before the buffet.

So, Boeing have chosen the parameter that should give the earliest warning.

[TyRod]

I would be quite surprised to get low speed buffet before the stick shaker...even above 20 000ft.....

[I-2021]

TyRod have you ever practiced approach to stall / stall rcovery in the Sim (hopefully) ?

[Avenger]

I would be quite surprised to get low speed buffet before the stick shaker...even above 20 000ft.... Do you actually operate the 737? this is a regular mandatory sim exercise. You may be surprised but Boeing will not be, thats why they make several references to this is their High Altitude Manoeuvring publications and the FCTM.

[sheppey]

I would be quite surprised to get low speed buffet before the stick shaker...even above 20 000ft....
Do you actually operate the 737? this is a regular mandatory sim exercise. You may be surprised but Boeing will not be, thats why they make several references to this is their High Altitude Manoeuvring publications and the FCTM.

During high altitude approach to stall recovery (737 Classic for example) there is no specific mention in the FCTM as to what airspeed you should attain before safe recovery to level flight. A clue is under Holding above 25,000 ft with no FMC. It recommends Vref40 plus 100 knots. eg 50,000 kgs would be 131 +100 = 231 knots. If the stall was due to mishandling in severe turbulence and during the subsequent recovery the aircraft is still in severe turbulence, then you should keep descending to reach recommended severe turbulence speed of 0.73 or 280 knots IAS before levelling out. Height loss will be about 3000 feet no turbulence - or 5000 feet to attain severe turbulence penetration speed.

[Avenger]

Can't recall making any reference to recovery techniques or speeds in my posts. Thread creep I think

[eckhard]

I would be quite surprised to get low speed buffet before the stick shaker...even above 20 000ft.....

From the 777/787 FCTM (my italics):

High Altitude Maneuvering, “G” Buffet


Airplane buffet reached as a result of airplane maneuvering is commonly referred to as “g” buffet. During turbulent flight conditions, it is possible to experience high altitude “g” buffet at speeds less than MMO. In training, buffet is induced to demonstrate the airplane's response to control inputs during flight in buffet.

Note:
Stick shaker is close to initial buffet for all weights and altitudes. Stick shaker activation may occur if the airplane is maneuvered beyond buffet.

Establish an airspeed of 0.81M to 0.84M. Induce “g” buffet by smoothly increasing the bank angle until the buffet is noticeable. Increase the rate of descent while increasing the bank angle to maintain airspeed. Do not exceed 30° of bank. If buffet does not occur by 30° of bank, increase control column back pressure until buffet occurs. When buffet is felt, relax back pressure and smoothly roll out to straight and level. Notice that the controls are fully effective at all times.

[Gauges and Dials]

I think there's terminological confusion here.

There's a flight region (combination of IAS, mach, density altitude, etc.) at which imposing a 1.3g load will bring the aircraft "near" a stall (ignore the meaning of "near" for a second).

Above 20,000 feet, buffet is a reliable indicator that you're getting into that region.

Below 20,000 feet, buffet is not a reliable indicator, and so the ADC and other magical contraptions shake the stick when you're getting into that region.

The ADC and other magical contraptions also display the amber band such that, irrespective of whether you're at high altitude or low, the top of the amber band means you're getting into that region.

In other words, the top of the amber band always means the same thing: you're getting into that region. Above 20,000 feet or so, the actual aerodynamic physics of the aircraft (i.e. buffet) and the amber band will give you indication at about the same time. Below 20,000 feet or so, the stick shaker and the amber band will give you indication at about the same time.

[eckhard]

Thanks G and D.
You have very eloquently explained what I tried to in #7

[Derfred]

Can anyone eloquently explain what brings on a Stick Shaker in a 737NG?

I don't think it's purely an IAS warning, and I don't think it's purely an AOA warning. Is it a matrix of the two?

[Established]

Derfred

Direct quote from 737 NG FCOM.

Natural stall warning (buffet) usually occurs at a speed prior to stall. In some configurations the margin between stall and natural stall warning is less than desired. Therefore, an artificial stall warning device , a stick shaker, is used to provide the required warning.

The stall warning "stick shaker" consists of two eccentric weight motors, one on each control column. They are designed to alert the pilots before a stall develops. The warning is given by vibrating both control columns. The system is armed in flight at all times. The system is deactivated on the ground.

Two independent, identical stall management yaw damper (SMYD) computers determine when stall warning is required based upon :

* alpha vane angle of attack outputs

* ADIRU outputs

* anti-ice controls

* wing configurations

* air/ground sensing

* thrust

* FMC outputs.

The SMYD computers provide outputs for all stall warning to include stick shaker and signals to the pitch limit indicator and airspeed displays and the GPWS windshear detection and alert.

Two test switches are installed in the aft overhead panel. Pushing either of these initiates a self-test of the respective stall warning channel. The No. 1 activates the Captain stick shaker, and the No. 2 activates the F/O s shaker. Either stick shaker vibrates both columns through column interconnects.

( With thanks to Boeing )

Hope that helps.