Why cage the standby gyro?

[Iflyamouse]

I've wondered for a while now what is the purpose of having the 'cage' function on standby gyro's. I used to think it had something to do with not letting the indicators "roll over" when the gyro isn't powered but that theory is out the window with the new Integrated Standby Indicators that are appearing in aircraft like the CRJ2/7/9. I thought it was to get it to wake up quickly ... but againt the ISI's don't need that either. Why do I need to have a cage function?

If you're not familiar, the new ISI's are a digital standby indicator. They have an integrated attitude indicator, airspeed indicator, slip/skid indicator, and altimeter. More importantly, they have no gyro (that I know of) ... yet they still have a cage button. So it appears that the cage function isn't related to the gyro at all??

Anyone ... anyone???

 

[wooferdog]

purely an educated guess here...

I would think that it is a remote gyro. It has to have a gyro somewhere, especially if it is a standby gyro. Maybe that casge button sends an electric signal to cage that remote gyro.

Gyro cage is usually for aerobatics, or to get it to erect straight-and-level against a known plane (in this case, the ramp).

At the place I used to be at, we would immediately tow the aircraft to the hangar right after the pax got off. Come to find out, we were not letting the attitude indicator on the right side "spin down". Accordingly, when we moved the aircraft, that poor little gyro was getting the $hit beat out of it. A cage feature would have helped.

just my $0.02 here.

If you find out more, I would like to hear.

Happy 4th!!!

 

[Waldom]

Any stand-by gyro has to have input from a gyro or AHRS. They also have to have a source of electrical power independent from the aircraft electrical system. This is done with a dedicated stand-by battery that kicks in when ship's power is no longer availiable AND THE BATTERY IS NOT CAGED. So if you leave the stand-by gyro uncaged on shut-down, the next crew will not have emergency power for the stand-by gyro availiable because the emergency battery will be discharged.

Must be that portion of your aircraft ground school was taught right after lunch.

 

[skydivinguy]

Actually, it has a lot to do with the currently popular "cage fighting"!

 

[CHQ Pilot]

It probably all depends on the ISI. I know that the I've flown an aircraft that had a cage button on the ISI, but we were told to never use it. Its purpose was to have a quick realignment phase if during flight there was a power interruption that caused the attitude info to be bad. I think it had something to do with bypassing a lot of filters and going straight to the AHRS to quickly get the information back. Other aircraft I've flown that have an ISI (that are from a different manufacturer) don't even have a cage funtion. So, it comes down to probably what the manufacturer says the cage function does.

 

[Gorilla]

I thought some caging was the act of correcting a precession in flight when you have a certain cheaper variety of gyro. Let's say you've done two turns in holding, and when you roll out into VMC, you notice the SBY AI still shows 4 degrees of bank. Cage it when you know you're wings level to kind of "reset" the system.

 

[bocefus]

Me thinks "caging" is an incorrect term for this procedure. The little knob thingy should say, "pull to erect". Caging is for protecting the gyro mechanism for shipment, excessive maneuvering, etc.

 

[AWACO]

The Thales Avionics ISI, as installed in CRJ2/7/9 & Global Express, uses only external Pitot & Static, Instrument Panel Brightness Control, Battery Bus, and LCD Brightness inputs. There are no external gyro or any moving parts for that matter. The ISI determins pitch by using (2) solid state Accelerometers and (3) solid state Gyrometers. The ISI determins airspeed altitude by using Pitot and Staic pressure monitored by Silicon Pressure Sensors.

Cage pushbutton:
The CAGE pushbutton, when depressed for more than 1 second initiates the CAGING function, which is used to cancel errors induced in the attitude display following unusual aircraft maneuvering or an upset. If in the circumstance a significant discrepancy is noted between the PFD attitude displays and the the ISI, depressing the CAGE pushbutton starts the CAGING function, lasting 10 seconds, which will cage the attitude display to zero.

Note:
The CAGE pushbutton does not correspond to the fast errect function of electo-mechanical horizons. The CAGE pushbutton has to be used when an attitude discrepency of more tha 1 degree is noted.

In short the CAGE function of the ISI is used to zero the attitude display its NOT for gyro wear or battery rundown on shutdown (it gets power from the 28v battery bus).

Useless fact - the ISI will fault out with an error message after being on without power interruption for more than 145 straight hours. So turn off your aircraft before leaving it in the desert!

-Enjoy

 

[ReportCanoa]

Any stand-by gyro has to have input from a gyro or AHRS. They also have to have a source of electrical power independent from the aircraft electrical system. This is done with a dedicated stand-by battery that kicks in when ship's power is no longer availiable AND THE BATTERY IS NOT CAGED. So if you leave the stand-by gyro uncaged on shut-down, the next crew will not have emergency power for the stand-by gyro availiable because the emergency battery will be discharged.

Must be that portion of your aircraft ground school was taught right after lunch.

Not so fast.

A typical standby attitude instrument has a self-contained gyroscope either mechanical or solid-state as is the case with the new digital types like the ISI. The only external inputs on the ISI are the air data hoses from the standby pitot and static.

The pull-to-erect/cage knob serves two purposes for a mechanical type, and that is to realign the gyro following an uncaged power-down or following manuver-induced precession. You pull the knob to erect (align), and twist and lock before turning off the battery master. When you shut the battery master, the power to the unit is removed and the gyros begin to decelerate and spin out of axis if they are not caged. This wears the gyro.

The 'cage' button on a digital type serves the same purpose in aligning, but there is no need to "lock" the gyroscope since there are no moving parts to wear.

The independent power supply is a requirement for part 25 certificated airplanes, however the battery bus is considered "independent" because it is powered directly from the battery(s). If you're down to battery power only, they should last about 30 minutes under ideal conditions. If the battery bus isn't powered, the gyros will begin to spin down and be unusable after five to ten minutes.

I ripped the following from the Canadian TSB report on Swissair 111 (note that the MD-11 meets both Canadian and US certification requirements):

[As stated above, regulations require that the standby attitude indicator be powered from a source that is independent of the aircraft's electrical generating system. In the case of the MD-11, the standby attitude indicator is powered from the battery bus, which is independent. However, there is no requirement for standby instruments to remain powered by an independent power supply separate from the aircraft electrical system and battery. Recent technological advances have been made in the area of independent standby instrumentation, and in providing for secondary navigation and communication capability. The TSB is assessing the safety impact of having an independent standby system in aircraft, that would provide flight crews with a "get-home package" in case of total electrical failure. Another potential benefit of such a system could be that it would allow for more options when de-powering an aircraft's electrical system in a smoke/fire event.
Following the SR 111 accident, Swissair decided to modify the standby flight instrument equipment in their aircraft. For their MD-11 aircraft, they chose to install a secondary flight display system (SFDS), which has a similar layout to the primary flight display in that aircraft. It includes attitude, altitude, airspeed and heading in one integrated display. In the event of a loss of primary aircraft electrical power to the unit, it has an auxiliary battery to supply power for a minimum of 45 minutes.]

I'm sure your airplane has a different setup, but not all airplanes are the same. Enjoy your lunch.

 

[bocefus]

I would agree with that except I haven't seen a lockable "pull to cage" knob in a long time. That's why me thinks that if it can't be locked, it is a pull to erect only. Pull to cage would be lockable.