Help me make sense of this.. HP or LP in the cockpit?!

[PalmettoZ]

The high pressure air outside the cabin is supplied through the pitot tube which gets full impact air pressure when the aircraft is in motion and going fast enough to be effective. The VSI, Airspeed Indicator and Altimeter all get feed from the pitot head. If it gets pluged the pressure is supplied from at static pressure port.

Check out AC 65-15A.

In other words, don't take my word for it, I've been known to be wrong sometimes.

Waddie

 

 

 

Hey Waddie-

 

Air pressure outside of the cabin in an upressurized aircraft is the same as the inside pressure, it is not higher. BUT it is influenced by opening a ram air valve or window for example by creating a lower pressure.

 

Also the altimeter gets its NORMAL input from the static port, not from the pitot tube. The airspeed indicator gets its input from both the Pitot tube and the static port.

 

The ATLERNATE position of the altimeter gets it input from inside the cabin, which is why if ram air or a window is open it will cause it to vary slightly from the NORMAL reading.

 

Vertical airspeed also only uses static pressure to give indication of the rate of change in altitude in Feet Per Minute. It does not use pitot input.

[proxlamus©]

Let me state the question that has been bothering me.. and it looks like some of you guys as well lol

 

4908. If, while in level flight, it becomes necessary to use an alternate source of static pressure vented inside the airplane, which of the following should the pilot expect?!

A: The altimeter and airspeed indicator become inoperative

B: The gyroscopic instruments to become inoperative

C: The vertical speed to momentarily show a climb << answer

 

4930. If while in level flight, it becomes necessary to use an alternate source of static pressure vented inside the airplane, which of the following variations in instrument indications should the pilot expect?

 

A: The vertical speed to momentarily show a descent.

B: The altimeter to read higher then normal << answer

C: The vertical speed to show a climb.

 

The description of the answer from the FAA says the following :

If the alternate static source is vented inside the airplane, where static pressure is usually lower then outside static pressure, selection of the alternate source may result in the following instrument indications:

1. The altimeter reads higher then normal

2. Indicated airspeed greater then normal, and

3. The vertical-velocity indicator momentarily shows a climb.

(H859, H931) - FAA-H-8083-15, Chapter 11; FAA-H8083-21

(H931, H758) - FAA-H-8083-25, Chapter 3; FAA-H-8083-21, Chapter 12

 

Jeesh a lot of typing. SO anyway.. thats where i'm getting my confusion. Hell.. look at the two different answers on the questions that are virtually identical!

[Guest Rolling Parts]

4908 is easy because A&B don't make sense when using Alt Static (but C is possible).

4930 is easy because the vertical speed will not show a climb, only a TEMPORARY climb.

[PalmettoZ]

Let me state the question that has been bothering me.. and it looks like some of you guys as well lol

 

 

 

 

 

The description of the answer from the FAA says the following :

 

 

Jeesh a lot of typing. SO anyway.. thats where i'm getting my confusion. Hell.. look at the two different answers on the questions that are virtually identical!

 

 

 

Hey Proxlamus-

 

I think the main point behind the questions that the FAA is trying to make here is that you realize that there will be a difference from the normal selection and the alternate, and to be familure with that difference before you get into a situation where you need to use the alternate source.

For what its worth, I would concentrate on why there is a difference, especially on any alternate or emergency procedure. By the way, I have an A@P license and a flight engineer license for 727 and 747 with thousands of flying hours. Had many a checkride and still turn wrenches in the Air Force reserves on the C17.

 

4908 They want you to realize that switching the sources produces this slight climb in vertical speed momentarily. although the plane is not actually climbing. Key word being mometarily since there is a slight difference in pressure until it equalizes.

 

4903 They want you to realize that there will be a difference showing on the altimeter because you now have a different source giving information going to it. Again, depending on how tight the cabin, or if a window is open or any airflow is coming in will determine that difference. Maybe this photo will help some. Hopefully I have not confused you more.

 

 

 

 

Also for what its worth, on a large jet where both the pilot and co-pilot both have altimeters, airspeed, and rate of climb indicators there is almost always a slight difference between the two sides. This is because of the fact that there is set of pitot tubes and static ports on both left and right sides of the aircraft.

[proxlamus©]

4908 is easy because A&B don't make sense when using Alt Static (but C is possible).

4930 is easy because the vertical speed will not show a climb, only a TEMPORARY climb.

 

 

 

I agree!! I have no problems answering the questions the way they are.. but after reading the description.. I was really thrown off by the reasoning behind the answer

[Guest Rolling Parts]

I agree!! I have no problems answering the questions the way they are..

 

 

Well,

just answer the test the way they want you to and pass the written.

Their reasoning is sound within the framework that they are presenting it.

 

I'm glad that they are still asking questions about "steam gauges" even though GPS/EFIS in unpressurized planes relegates the old gauges to non-primary for flight...