Static Port

[cweisheng]

I am not a pilot but working in the aerospace industry.

I read that some airplanes have alternative static port/source in an unpressurized part of the cabin, and that the instruments could be switched to read from this.

If this is good enough, why are external static ports (on fuselage, tubes etc) required, esp since they can ice up?

If this is not good enough, why not? I would like to understand the difference in physics behind having the port internal and external.

Thanks alot!

[Mad (Flt) Scientist]

There's no guarantee that a sensor inside the airframe is really sensing the true static air pressure. You'd have to prove that (for certification purposes). That wouldn't be trivial.

Even if in steady state the pressure were to equalize, during rapid climbs or descents, it will take time for the static pressure inside to catch up with changes outside. that means the altitude and airspeed would misread.

This effect is even seen with existing sensors - an aircraft with a trailing cone (for experimental purposes) has to be very mindful of this during dive testing. And that's with much less volume to equalize than a large chunk of aircraft volume.

[cweisheng]

Thanks for the insight. So, it all depends on the pressure inside equalizing with the outside, and there is some time response.

I am curious about the pressure equalizing; I was asking myself why would there be any guarantee that the pressure inside the fuselage attains the static pressure of that altitude?

- assuming a solid box with air in it, in this case, a fully sealed box that is not like an aircraft where the rivets etc might not be air tight
- when there is some (not drastic) change in altitude, the difference in pressure is not large between the inside and outside of the box
- and it depends on the material of the box, which might alleviate some of the pressure differential (?)

By the way, you mentioned about certification. Would you happen to know what those tests involve?

Thanks. Great help.

[CJ Driver]

The external static ports are placed where they will be (relatively) immune to airspeed and turning motions, so that they give a true indication of the air pressure. This takes careful analysis and often testing by the airframe designer, but can determine altitude to within a few feet.

An alternate source is generally inside the fuselage structure somewhere, and is not corrected for any of those effects - it will show errors due to the speed of the aircraft, the pitch attitude, any yaw, whether the cabin vents are open or closed, and so on. The alternate air source may have errors of over 100 feet relative to the true air pressure. The alternate source is therefore an emergency facility, not for normal flight.

[Mad (Flt) Scientist]

The relevant regulation (for FAA Part 25 at least) is "§25.1325 Static pressure systems"

Incidentally, this pretty much answers the original question - why are external ports used - as the reg explicitly requires external ports:

(a) Each instrument with static air case connections must be vented to the outside atmosphere through an appropriate piping system.

The regulation also requires that the system be designed, as CJD says, to minimize interference. The reg also requires that the installation be proven against icing, to the point about susceptibility to icing.

In terms of HOW this is done in cert, the basic approach is to compare the sensed pressures from the production air data sensors to a known reference. This reference is typically provided by a trailing cone which allows for a static pressure sensor a significant distance behind the aircraft, where the influence of the aircraft on static pressure is assessed as minimal. The aircraft is flown in a range of flight conditions and configurations, and the differtences between the reference trailing cone pressure and the production system pressures are termed the PEs or Position Errors.

A set of corrections are then devised, called the Position Error Corrections (PECs) or Static Source Error Corrections (SSECs) which may be supplied to crew as a set of correction charts, or directly implemented into the systems as an electronic correction before the data is presented to crews.

For systems where an electronic correction is not possible, physical changes can sometimes be made to the installation (changing the location or detailed shape of the sensor, or of the fuselage near the sensor) to aerodynamically correct the pressure value.

[cweisheng]

Thanks! Really helped me out here.

[grounded27]

I read that some airplanes have alternative static port/source in an unpressurized part of the cabin, and that the instruments could be switched to read from this.

Can anyone think of an aircraft that has a internal static port used for instrumentation because I can not. Think he may be confusing this with the internal port for a cabin pressure controller or sensor for O2 mask deployment.

[NSEU]

Think he may be confusing this with the internal port for a cabin pressure controller

Only useful when the aircraft is depressurised ;)

[NutLoose]

Can anyone think of an aircraft that has a internal static port used for instrumentation because I can not. Think he may be confusing this with the internal port for a cabin pressure controller or sensor for O2 mask deployment.

The likes of the Unpressurised Piper Pa 28 and the rest of the unpressurised range has one under the lip of the instrument panel, it is very similar to the fuel drain in design, on the likes of a Cessna 152 you break the glass on the VSI in an emergency.
Though as said there is some discrepancy between the outside port as the internal one so it is not as accurate.

The two I mention need an alternate source as they only have one external static port unlike most aircraft that have two.
Even on aircraft with two ports one on either side of the airframe, if one is blocked for whatever reason the system will lose accuracy because the ports on either side will normally compensate against each other for airflow etc over them.
Where an aircraft has two systems i.e pilot and co pilots and has 4 ports, two either side above and below each other, the system will be cross wired, i.e the upper port on the pilots system will cross connect to the lower on the other side and vice versa so both systems sense from an upper and lower port on either side so read equally between both systems, if that makes sense.

This shows describes the TBM system from its manual

http://www.smartcockpit.com/download.php?path=docs/&file=TBM_850-Air_Data_System_and_Instruments.pdf


Hope that helps.

[grounded27]

Just worried that the OP had an idea that an internal static port was in any way useful on an aircraft that can achieve the ROC or ceiling level of a jet powered aircraft. He referred to it it as unpressurized cabin pressure.

[A Squared]

Well, the OP said "in an unpressurized part of the cabin", I took that to mean "in an unpressurized part of the fuselage"

[noughtsnones]

Some engine controllers measure static pressure under the engine cowl. Measurement in this area may have a very low risk of calibration problems due to common mode threats e.g. insects, their nests, volcanic ash dust, water and ice.


Calibration of these sensing locations is though a complex issue due to variations in (performance of) cowl ventilation inlet outlets over the flight regime and pressure variation due to pneumatic starter motor exhaust and thrust reverser operation.


In recent years, higher bandwidth digital data buses (ARINC 664) have been used to provide engine sensor data measurements and validity (calibration threat flags) back to the aircraft. I believe that this data is used to determine whether the aircraft sensor data is itself out of calibration (e.g. due to ice) I do not know of any aircraft that today use this data for its instruments.


As long as the engine sensor data calibration threats can be managed, there is no reason not to expect more usage of these data in aircraft systems in the future, though the costs of such systems complexity need to be overcome by some overt safety or commercial need.

[Australopithecus]

Back in the day a friend departed in poor vis in a Cessna 402 which had an alternate static port in the nose baggage compartment. The static source selector valve had previously failed to the alternate source, undetected.

After take-off a baggage door popped open, ejecta damaged the left prop, and in the subsequent NDB approach to the Arctic strip they descended into the snow pack while indicating over 500' AGL. The two pilots survived, just.

The altitude error combined with the ambient conditions almost killed them.