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Some background information: "Static pressure is the measurement of the ambient barometric pressure at the aircraft’s CURRENT location AND CURRENT Altitude. The Static Port is located in a position on the aircraft that will not be affected by air flow as the aircraft moves through the air. This is typically on the side of the fuselage but can also be on the back side of the Pitot Tube or any other number of locations, it varies by the aircraft. Again smaller aircraft will typically have one Static Port, larger aircraft with redundant systems will have two."

After reading through that paragraph, I am wondering as to why smaller aircraft typically have one static port, whilst larger aircraft have two or potentially more static ports.

SillySquishy
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    Actually most smaller aircraft also have two, the other being pilot controlled and called "alt static" that provides an alternate static source (usually in cabin) when the external one gets plugged. – Ron Beyer Dec 27 '19 at 13:49
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    And some also have 3. My Cessna 177 has 3 ports, one in each side of the empanage and an alt-static port in the cabin. – Ron Beyer Dec 27 '19 at 14:10
  • You must add sources for your citation. – Manu H Dec 28 '19 at 12:43
  • Trainers like my Cherokee save money by having a combined pitot/static vane. More capable aircraft often have redundant static ports—usually one on each side of the fuselage—because they fly in IFR weather. Often, aftermarket autopilots will have their own static port. – JScarry Dec 28 '19 at 16:11

1 Answers1

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Your quote already answers your question:

larger aircraft with redundant systems will have two.

The key is redundancy. You want to have at least two independent systems to measure altitude and airspeed so if one fails, the other still provides good data. Of course redundancy is also a good thing in a small aircraft, but it always comes at a cost: complexity. With larger aircraft the added complexity and therefore maintenance cost does not make such a large difference any more therefore tipping the balance in favor of redundancy.

For passenger airliners with potentially hundreds of people on board, safety is far more important. They usually have even more than two static ports. E.g. the Boeing 737:

Air Data

The pitot static system is comprised of three separate pitot probes and six flush static ports. Two pitot probes and four static ports interface with the air data modules. The remaining auxiliary pitot probe and alternate static ports provide pitot and static pressure to the standby instruments. The auxiliary pitot probe is located on the first officer’s side of the airplane.

The air data modules convert pneumatic pressure to electrical signals and send these data to the ADIRUs. Each pitot air data module is connected to its on–side pitot probe; there is no cross connection. The air data module connected to the Captain’s pitot probe sends information to the left ADIRU, while the air data module connected to the First Officer’s pitot probe sends information to the right ADIRU. The remaining air data modules are located at the balance centers of the Captain’s and First Officer’s static ports. The air data module connected to the Captain’s static ports sends information to the left ADIRU, while the air data module connected to the First Officer’s static ports sends information to the right ADIRU.

(Boeing 737 NG FCOMv2 10.20.13 Flight Instruments, Displays - EFIS/Map System Description, emphasis mine)

This results in three independent measurements of altitude and airspeed:

  1. Captain's side
  2. First Officer's side
  3. Standby Instruments
Bianfable
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