changing altimeter setting while in AP-BALT mode

[HILOK]

when in BALT, shouldn't the autopilot correct the aircraft's vertical path to maintain barometric altitude, in case the altimeter barometric setting is changed? at least below transition level? because it doesn't.

e.g. if you fly at 1000ftMSL, engage AP-BALT and then increase your baro setting, the altitude indication will rise. my understanding is that the autopilot should start descending to catch the 1000ftMSL. but it seems the hornet's AP is currently set to a fixed reference qnh. is this correct?

[HammerUK9]

I guess it depends on the system under the hood. If it's simply referencing the static pressure as measured at the time of engagement, then the setting on the altimeter is irrelevant.

[BIGNEWY]

I have not seen it mentioned anywhere. We would need to see evidence of this for our hornet. 

 

thanks

[HILOK]

of course, thanks. i am pretty sure though other auto flight systems exhibit this kind of behavior

[imacken]

2 hours ago, HILOK said:

when in BALT, shouldn't the autopilot correct the aircraft's vertical path to maintain barometric altitude, in case the altimeter barometric setting is changed? at least below transition level? because it doesn't.

e.g. if you fly at 1000ftMSL, engage AP-BALT and then increase your baro setting, the altitude indication will rise. my understanding is that the autopilot should start descending to catch the 1000ftMSL. but it seems the hornet's AP is currently set to a fixed reference qnh. is this correct?

Surely you are just setting BALT to maintain level flight, you're not giving it a target altitude, you're just asking it to hold your BALT no matter what it is.

Seems correct to me.

[Stearmandriver]

In every aircraft I've ever flown, if you engage altitude hold and then change the altimeter setting, the plane will climb or descend to recapture the altitude, as the OP suggests.  No idea if an F-18 does that, but it's a reasonable question because if it does not, it's the exception rather than the norm.

Edited March 3, 2021 by Stearmandriver

[imacken]

Yep, I take your point.  It would be interesting to know which one is correct behaviour, or if the Hornet is somehow different.

[TimRobertsen]

21 hours ago, HILOK said:

when in BALT, shouldn't the autopilot correct the aircraft's vertical path to maintain barometric altitude, in case the altimeter barometric setting is changed? at least below transition level? because it doesn't.

e.g. if you fly at 1000ftMSL, engage AP-BALT and then increase your baro setting, the altitude indication will rise. my understanding is that the autopilot should start descending to catch the 1000ftMSL. but it seems the hornet's AP is currently set to a fixed reference qnh. is this correct?

 

I had the same thought flying earlier today; unless there is a some kind of compensating system, the aircraft should climb or descend when you change the reference-pressure.

Off to the natops!

Edited March 4, 2021 by TimRobertsen

[Cruizzzzer]

On the Boeing 737, while pressing "ALT HOLD" (= BALT), the A/P maintains the uncorrected barometric altitude, so i.e. ALT HOLD is NOT coupled to the altimeter setting, nor altitude will change while changing your altimeter setting. So, having set ANY altimeter setting is only an indication, but has NO effect on the autopilot itself, as it "senses" the barometric altitude "on its own" (ADC)...

 

I guess that every aircraft shows this behaviour...

 

Edited March 20, 2021 by Cruizzzzer

[Cruizzzzer]

the NOTE also points this out...

[Machalot]

On 3/20/2021 at 1:02 AM, Cruizzzzer said:

I guess that every aircraft shows this behaviour...

The Viggen altitude hold autopilot responds to changes in altimeter setting. 

[Cruizzzzer]

2 hours ago, Machalot said:

The Viggen altitude hold autopilot responds to changes in altimeter setting. 

good to know...

[maxTRX]

On 3/20/2021 at 1:02 AM, Cruizzzzer said:

On the Boeing 737, while pressing "ALT HOLD" (= BALT), the A/P maintains the uncorrected barometric altitude, so i.e. ALT HOLD is NOT coupled to the altimeter setting, nor altitude will change while changing your altimeter setting. So, having set ANY altimeter setting is only an indication, but has NO effect on the autopilot itself, as it "senses" the barometric altitude "on its own" (ADC)...

 

I guess that every aircraft shows this behaviour...

 

 

 

I guess in an airliner you have to set the V/S rate before plane responds (?).

In a fighter jet I 'think' the baro setting change would initiate the response immediately, at a fixed rate I imagine?

Where have all the Hornet drivers gone?  None of them respond to any q's anymore 

[Machalot]

1 hour ago, Gripes323 said:

 

I guess in an airliner you have to set the V/S rate before plane responds (?).

According to what was stated above in the B737 manual, the aircraft holds a constant uncorrected altitude. That is the raw altitude measurement prior to the altimeter pressure correction. There is no situation in which changing the altimeter pressure setting would cause the autopilot to change altitude. 

 

1 hour ago, Gripes323 said:

In a fighter jet I 'think' the baro setting change would initiate the response immediately, at a fixed rate I imagine?

Whether it would respond at all depends on which altitude measurement the autopilot uses. What climb/descent rate profile it would use depends on the specific design of the autopilot in that particular jet. 

Edited March 22, 2021 by Machalot

[Cruizzzzer]

1 hour ago, Gripes323 said:

 

I guess in an airliner you have to set the V/S rate before plane responds (?).

In a fighter jet I 'think' the baro setting change would initiate the response immediately, at a fixed rate I imagine?

Where have all the Hornet drivers gone?  None of them respond to any q's anymore 

You could get out of the "ALT HOLD" by pressing MCP speed, i.e. changing the vertical mode, thus the B737 would maintain speed (according to thrust lever setting it would climb/descend).

 

During "ALT HOLD" changing to V/S nothing will happen if you hadn't chosen/selected another altitude.

 

And, phew, guess I need to test that on the hornet... it just popped in my mind when I read the thread title how it works with the boeing...

[Stearmandriver]

That's funny, because I've been flying 73s for 7 years now and could have sworn it did.    I guess since we spend so much more time in VNAV, I'm thinking of that behavior.  But other airliners I've flown will very definitely recapture the corrected altitude.  The 737 is an anachronism, remember... this is the plane that can't even fly a coupled missed approach with one autopilot.

 

That said, the Hornet NATOPS doesn't seem to specify, so I'm not sure, short of feedback from someone who's done it, how we could determine correct behavior either way.  But plenty of aircraft hold corrected altitude, and will chase it. 

[maxTRX]

Corrected v uncorrected

Everyone goes to 29.92 above 18k. Any time the controller gives the pilot an altitude that passes that transition alt. on descent, the pilot would set the local setting immediately before setting the new ALT HOLD... isn't this how this works?

Edited March 22, 2021 by Gripes323

[TimRobertsen]

47 minutes ago, Gripes323 said:

Corrected v uncorrected

Everyone goes to 29.92 above 18k. Any time the controller gives the pilot an altitude that passes that transition alt. on descent, the pilot would set the local setting immediately before setting the new ALT HOLD... isn't this how this works?

 

I thought it was standard to switch to "standard atmosphere" at 10.000ft.

[maxTRX]

1 hour ago, TimRobertsen said:

I thought it was standard to switch to "standard atmosphere" at 10.000ft.

 

For North America it's 18k.  Otherwise it varies... a lot depending on where you fly, even in one country it could vary from one area to another, from what I gather.

[Stearmandriver]

There's no need to turn off the autopilot passing through transition level / altitude.  When climbing or descending, you're not in altitude hold, you're in a different vertical mode like vnav, vertical speed, level change etc.  Alt Hold is (sometimes) armed, but not active until you reach the selected altitude. 

Edited March 23, 2021 by Stearmandriver