Boeing (747-400) auxiliary static system?

[RandomPerson8008]

Hello,

In a 744 equipped with only 2 ADCs (L and R), what function do the auxiliary static lines serve? What systems and/or flight instruments does this part of the static system provide information to? In 3 ADC aircraft, the auxiliary static ports supply the Center ADC. However in 2 ADC aircraft, I cannot determine what the aux static information is used for.

The static ports for the auxiliary static system appear to be on the aft portion of the 4 pitot tube masts on the sides of the forward fuselage and appear to be heated independently of the primary static ports. If the heating element for the auxiliary static ports fail, a "HEAT P/S L AUX" or "HEAT P/S R AUX" occurs. The aircraft can then be dispatched with an MEL but "operations into visible moisture or known or forecast icing conditions are prohibited". If the failure occurs in-flight, the non normal checklist in an aircraft equipped with only a L and R ADC simply states "Flight in icing conditions may result in erroneous flight instrument indications".

Here is the pitot/static diagram for a 744 equipped with only 2 ADC's. There is no indication of where the auxiliary static information goes. Asking because we have a training scenario involving this EICAS message and given the lack of information it is challenging to make an accurate assessment of the threat.

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/627x836/image0_8__780f55d80bb24cbbe66d5d6c9a64823a25071f68.png

[Intruder]

Standby altimeter/airspeed or standby Integrated Flight Display.

[RandomPerson8008]

Intruder, thank you for the response!

Is it an oversight then, of the attached system diagram from FCOM vol 2 that the Standby altimeter/IFSD appears to receive static from the "alternate static source", not the "aux static"? I realize that these diagrams are often very dumbed-down and simplified, and therefore aren't always completely accurate.

With that said, the diagram makes it appear that the IFSD/standby instruments receive pitot info from the lower left and right pitot tubes and static from the separate, flush mounted "alternate static" ports on the middle forward section of the fuselage, rather than the pitot-mast mounted static sources that comprise the auxiliary system.

[STBYRUD]

I can't speak for the 747, but on the 777 as well as the 737 the aux static ports on the pitot probes are unused, and this is mentioned in the AMM systems description. It may well be the same on the 747 then.

[RandomPerson8008]

Thank you STBYRUD. It is strange that the QRH says that flight in icing conditions may result in erroneous flight instrument indications with a heat P/S aux EICAS advisory in a 2 ADC airplane. The checklist is more lengthy in the 3 ADC airplanes. I would probably need the AMM for the answer. Maybe the auxiliary pitot static heating system does not exactly correlate with the auxiliary static system. I think Intruder may be right because the 3 ADC checklist does make mention of erroneous standby instrument indications specifically.

[punkalouver]

I can only guess. It appears that you are saying that some 747’s have 2 ADC’s and some have 3 ADC’s. Perhaps it was an option for the purchaser to choose two or three.

For those who choose three, it is plumbed to the third ADC, it is appropriately plumbed. For those who choose two ADC’s, the static system is still installed but not used. However, it could be used in the future if a third ADC is ever installed.

[RandomPerson8008]

Thanks for the response! Yes, 2 ADC vs 3 ADC was a customer option. My employer has some of both. I did not think of the possibility of retrofitting a 3rd ADC later, interesting point that would justify the existence of the auxiliary static lines in the 2 ADC ships.

[tdracer]

I can't speak for the 747, but on the 777 as well as the 737 the aux static ports on the pitot probes are unused, and this is mentioned in the AMM systems description. It may well be the same on the 747 then.
No, on the 747-400 (and -8) the primary static sources are the ports on the pitot probes. I honestly don't know why they keep changing between the pitot probe statics and the flush mounted "saltshaker" statics on the various aircraft, but it's clear that they do. Even the 757 and 767 are different (757 uses the saltshaker style, 767 uses pitot static) even though the two aircraft were co-developed and certified using common avionics and a common type rating.

BTW, apparently Boeing decided that building both 2 ADC and 3 ADC configurations for the 747-400 was more trouble than it was worth - the 747-8 has the 3 ADC configuration basic.

[punkalouver]

Thanks for the response! Yes, 2 ADC vs 3 ADC was a customer option. My employer has some of both. I did not think of the possibility of retrofitting a 3rd ADC later, interesting point that would justify the existence of the auxiliary static lines in the 2 ADC ships.

Or maybe for minimizing manufacturing cost, they just build them all the same ;ie. all have the aux static system plumbing), but only have it used if the optional equipment is installed.

I had a recent separate internet conversation about a totally different aircraft type that has unnecessary equipment installed on a smaller version of that type strictly for manufacturing commonality. One version is less than 12500 lbs making the equipment unnecessary.

[dixi188]

Lots of aircraft are fitted FOR but not WITH various systems. Same as cars.
It makes the production process easier as any changes down the line are catered for.

[Intruder]

Standby altimeter/airspeed or standby Integrated Flight Display.
My mistake. I confused aux static and alt static.

With only 2 ADCs, aux static do nothing.

[gipilot]

A quick question. I would just want a clarification on all of this. So the primary static ports for the ADC LEFT, CENTER AND RIGHT are in the pitot tubes. So what are the upper and lower salt shakers for up front? Are they interconnected with the pitot tubes?

Another one….The salt shakers on both sides of the tail, are those the ALTERNATE static ports?

cheers mate

[CCA]

Auxiliary pitot #1 & #2 plus the local static ports located near the lower forward fuselage area of the stabiliser are used for elevator feel.

From memory the classic didn’t have the static ports at the stab.

[gipilot]

Yes, I figured it out. On 2 ADC aircraft the AUX plumbing is just there and not used by any system. On 3 ADC a/c obviously it’s for the center ADC.

On the left forward side the lower saltshaker is a CABIN static source and the upper left and right salt shakers are ALT static sources for the stby Altimeter and Speedometer.

The saltshakers in the tail are for the ELEVATOR FEEL computer.