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.

 

[Singlecoil]

Now...are you flying a C-172? Go to the Performance charts in Section 5 for Airspeed Calibration using the Alternate Static Source. In the FAA configuration, is the pressure higher or lower. Well the inside cabin pressure is higher because the airspeed corrections all move lower. We all know that the Airspeed is measuring the difference between RAM and static pressure - if the speed is "indicating" less then there must be less of a difference (i.e. the pressure is higher inside).

Huh? I don't have the chart in front of me, but if the corrections move lower, that is because the airspeed is erroneously indicating high. Pull the alternate static and you will notice airspeed higher than normal, altitude higher than normal, and VSI momentarily indicates a climb. All indicate lower than static pressure in the cabin. Read the airspeed indicator, apply the correction from the chart (to lower the figure you are seeing) and be on your merry way.

 

[supsup]

Compass Turns

Try this CEO. Take the obs or adf and put north on top. Now I use UNOS, Undershoot North Overshoot South. Only be concerned with what heading your turning to and from what direction. Back to the OBS, super impose 30 at North, 20 at 330, 10 at 300, 0 at 270. For example turning right to a heading of north you would roll out on a heading of 330 (Undershoot by 30 degrees). For south heading it works the same, 30 at 180, 20 at 150, 10 at 120 and 0 at 090. Turning right to a heading of 180 would require the mag compass to read 210 degrees.

supsup

 

[midlifeflyer]

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.

I always wondered about that. I got my instrument rating about 10 years ago and my CFII taught me both methods. I always found compass turns easier and did them exclusively on my checkride with no problem.

The mnemonic I ended up using instead of the standard "South Leads, North Lags" is the same ANDS we use for acceleration compass errors, but in this case its,

Anticipate North, Delay South

 

[atldc9]

Believe it or not, I had to do compass turns on my last FAR 121 PIC checkride. The Boeing 717 has a standy instrument, called an ISIS, that provides attitude, altitude, and airspeed information in the event all other cockpit information is lost. It is a electronic presentation that is totally independent of all other cockpit systems. BUT, it does not provide heading information!!!! If ypu find yourself in a situation where you are down to the ISIS, which is very unlikely, like 10 to the 999th, you would be stuck doing compass turns. When flying one of the most advanced flight decks in the air today, the last thing you expect to do is compass turns. What was standard rate again?

 

[PennEagle]

Compass Turns

Well, here I am looking out the window at probably 2400RVR and VV001. So..... it's either go and do my taxes or wax lyrical on compass turns ......

I use "never see north" and "sail through south" when teaching compass turns. So, if bloggs is looking to turn from 360 to, say, 240, he's going to sail through 240 to a heading 245 (5 deg correction at my latitude) to settle out on the correct magnetic heading. Likewise, if (s)he wants to turn from 180 to, say, 060.... (s)he will never see 060 on the MC and should turn to a heading 080 (again, at my latitude).

Corrections are:

North ... ... ... Lat. + 7 deg (1/2 angle of bank)
330 or 030 ... 2/3 Lat. + 7 deg
300 or 060 ... 1/3 Lat. + 7 deg
270 or 090 ... Nil
240 or 120 ... 1/3 Lat. - 7 deg
210 or 150 ... 2/3 Lat. - 7 deg
South ... ... ... Lat. - 7 deg

An excellent exercise is to fly the plane yourself, and let the student really study how the MC behaves on a full standard rate turn. The MC can appear to be stationary for an awefully long time in certain heading/direction combinations, and at other times be doing Mach 2. Students who glance only briefly at it without understanding how it behaves can roll out early or late.

Finally, here's a fun question from my CFII oral before I go back to watching the snow ..... You are S&L heading 270 when you lose all your instruments in IMC, except the MC. The AP is on and you decide to climb at a constant IAS to known clear conditions. What does the MC read during the climb, a) West, b) South of West or c) North of West.