Alternate static source and compass q's

[ceo_of_the_sofa]

First things first,
Why is the pressure inside the cabin lower than outside (as is evident when using the alt. static source?), assuming you're flying, and not sitting on the ground with the doors open?
(this is for non-pressurized aircraft)
This is how I understand it, and it's kind of confusing. If, according to the venturi principle, higher velocity (outside the cockpit) means higher dynamic pressure it should also mean lower static pressure as a result. So, shouldn't the static pressure be higher inside the cabin, where dynamic pressure is very low? What gives?

Also (here's a double whammy for ya, CFIs) when turning TO/FROM Northern headings, MC lags, and turning TO/FROM Southern headings it leads...shouldn't we just concern ourselves with the DG and not worry about it? The heading indicator is not affected by any of the dipping, is it? So provided it set correctly every 15 minutes or so, we can theoretically use the compass solely for correcting the DG?

Discuss...

 

[groundpointsix]

Well, I've spent the past hour trying to find an explanation for you on the alt. static source question; I can't find one. Every textbook I have simply expects you to just accept that this is how it is. You've got me curious now. I hope someone can provide an explanation.
As for your DG question; you're right, as long as you update to your compass every couple of minutes you don't need to worry about compass errors-- until your DG fails and you have to navigate by compass alone. That's the main reason you have understand the errors of the compass. Additionally, you cannot accurately set your DG unless you can accurately read your compass. An understanding of what your compass is doing will prevent you from trying to set your DG to an incorrect setting. (Though I think common sense would dictate waiting for your compass to settle down before trying to match the DG to it.) Of course, if you have an HSI, you never have to set the DG again!

 

[Doozer]

CEO:

Perhaps if you think about how the airfoil develops lift; the area of low pressure is where the airflow is faster. With respect to the alternate static source, the higher velocity airstream over the fuselage creates an area of low pressure, which influences the pressure inside the cabin as well.

With respect to the wet compass/DG question, as long as the magnetic compass has stabilized & you take the compass correction factor into account, I agree with you. From a training standpoint, however, one should know how to work with the wet compass if the gyro fails.

 

[ceo_of_the_sofa]

CEO:

Perhaps if you think about how the airfoil develops lift; the area of low pressure is where the airflow is faster. With respect to the alternate static source, the higher velocity airstream over the fuselage creates an area of low pressure, which influences the pressure inside the cabin as well.

That's the explanation I was looking for!!! High velocity over the cockpit creates high dynamic pressure (low volume, in respect to Bernoulli's principle) and low static pressure, which in turn effects the cockpit through the various leaks (sucks the pressure out, sort of)...whew...OK

With respect to the wet compass/DG question, as long as the magnetic compass has stabilized & you take the compass correction factor into account, I agree with you. From a training standpoint, however, one should know how to work with the wet compass if the gyro fails.

So, from what you and checkpointsix have said, the concept is mainly reinforced to cope with DG failures, and to cement why compass may not read accurate during certain periods? Now, that begs another question, gentelmen (and ladies)...How is HSI immune from falling prey to the magnetic dip? Since it's slaved to the MC, shouldn't immitate everything MC does? (unless it's in slave free mode?)

 

[172driver]

I can tell you how the HSI in the Seminole works. Can't speak for other planes but I assume they are similar.

The HSI is, in fact, not slaved to the compass at all. It is an electric instrument which relies on two sources of information.

There is an electric gyro behind the baggage compartment which acts like any other gyro except that it must send its information electronically up front to the HSI. Also, there is a flux valve installed in the left wing which senses and sends magnetic information to the HSI...like a compass but not the compass you're used to seeing. So, the HSI is really slaved to the flux valve and remains stable and upright because of the electric gyro.

If the gyro fails, you will have an HSI subject to the same lead and lag errors as the mag compass. Fun stuff.

If the flux valve fails, you will need to select free mode and will be left with a run of the mill DG that must be set often to counteract precession.

 

[bobbysamd]

Compass errors

[W]hen turning TO/FROM Northern headings, MC lags, and turning TO/FROM Southern headings it leads...shouldn't we just concern ourselves with the DG and not worry about it? The heading indicator is not affected by any of the dipping, is it? So provided it set correctly every 15 minutes or so, we can theoretically use the compass solely for correcting the DG?

What are you going to do when you lose your suction pump and your HI and AI go south in a hard IFR? There is no if. You will lose suction at some point, even if only on your instrument ride. Therefore, what will be your heading, primary wings-level reference, and turning reference?

(I would declare and request vectors to the nearest airport with an ILS.)

Aside from that, don't sell the wet compass short. It is the only instrument that is independent of any other system in the aircraft. You can still turn to headings with reasonable precision if you understand compass errors and with timed turns.

Memory aid for compass turns: The South leads, the North lags, save your Confederate money.

Hope that helps. By the way, compass turns and timed turns to headings are very satisfying to pull off once you get the hang of them.

 

[ceo_of_the_sofa]

I am by no means discounting the importance of the MC, I just wanted to fully understand its purpose and relationship to DG. Since we're on the subject of MC turning errors, how much do I undershoot when turning to/from North and just how much do I overshoot turning South?

 

[bobbysamd]

Leading compass turn rollouts

Since we're on the subject of MC turning errors, how much do I undershoot when turning to/from North and just how much do I overshoot turning South?

I recall that a good rule of thumb was something like 30° plus half your bank angle for South turns and adding half your bank angle to the heading you want for North turns.

I don't have my Instrument Flying Handbook at hand. Hope this helps a little more.

 

[Doozer]

CEO:

It's kind of humorous to me, but the FAA, through the practical test standards (PTS), doesn't test us on what really happens with the magnetic compass--only timed turns.

In our primary & instrument training, the Navy & Marine Corps taught us that the lead/lag of our wet compasses were approximately equal to our latitude. The mnemonic was "stay out of the north; go through the south."

Therefore, at 35-degrees latitude, the lead/lag would be be a proportion of that 35-degrees as it related to how much our rollout headings deviated from east/west to north/south plus/minus half our angle of bank.

In other words, at 35 degrees latitude on a rollout heading of 045 or 315 the lead would be 17.5 (half of the latitude) plus half of the angle of bank during the turn; on a rollout heading of 135 or 225, one would fly through the rollout heading or 135 or 225 by an additional 17.5-degrees minus half of the angle of bank used for the turn.

Hope this helps to clarify magnetic compass error.

 

[ceo_of_the_sofa]

Thanks, Doozer (and others), very helpful, and precisely what I was looking for.