The pitot-static system and blockages

Navigation · Chapter 20 · DGCA CPL and ATPL

The three pressures, the pitot tube and static vent, and what each blockage does to the ASI, altimeter and VSI.

Remember

What is static pressure, really?

What air pressure is

  • Air is not empty • Air is made of tiny molecules, far too small to see • They fly about at around 500 m/s, bouncing off everything they meet
  • Pressure is the bumping • Every hit is a tiny push on the wall • Billions of hits a second add up to a steady push: pressure • At sea level: 1 013 hPa, about 1 kg on every cm²
  • The same from every side • The molecules fly every way, so they push the same on every face • Hold a plate in still air: equal teal arrows all round • This is static pressure (S)
  • Less air higher up • The air near the ground is squeezed by all the air above it • Climb: fewer molecules, fewer hits, S falls • 10 000 ft: 697 hPa; 40 000 ft: 188 hPa, under a fifth of the air • Drag the height and watch the dots thin out

What extra push comes from moving through the air?

Moving air: dynamic pressure

  • Move through the air • Fly at 150 kt and the air rushes at you at 150 kt • It is the same as standing still in a 150 kt wind: the relative airflow
  • An extra push on the front • The molecules now hit the front harder and more often (pink flashes) • That extra push, only from moving: dynamic pressure (D) • Count the hits: the front gets far more than the back
  • Stop the air in a tube • An open tube facing the flow: the air going in is brought to a stop • Its motion turns into pressure: inside feels S + D • A hole in the side feels only S
  • Twice the speed, four times D • D grows with the square of the speed • Drag the airspeed and watch D in the key
  • Thinner air, less push • Higher up there are fewer molecules to hit you • Same speed, less D: about 27 hPa at 150 kt and 10 000 ft • Drag the height

What does the pitot tube actually feel?

Three pressures

  • Static: all round • On the Cessna 172: a flat static port on the left side of the fuselage • The air slides past it, so it feels only static pressure (S)
  • Dynamic: from moving • The pitot tube hangs under the left wing on a mast • The air rams into its open mouth and stops: S + dynamic pressure • Watch the pink streaks stop dead in the mouth
  • Pitot = S + D • The open mouth feels both: pitot = S + D • The static port feels S only • Two lines go in: the ASI works out D = pitot − static
  • Four times • D = ½ ρ V²: twice the speed, four times D • Drag the airspeed: watch D in the key

How is the pitot-static head built, and why?

Pitot tube and static vent

  • Facing the air • Under the left wing, open end faces the airflow, parallel to the aircraft's axis • Held down on a mast, clear of the slow air next to the wing (the boundary layer)
  • Static: a separate port • The Cessna takes static from a flush port on the fuselage, at right angles to the flow: S only • Some aircraft put static holes on the side of the pitot tube instead: a pitot-static head
  • Heater and drain • An electric heater coil stops icing (the head is taken apart for you) • A drain hole lets water out (costs a little pressure, allowed for)
  • Why a flush port • A flat port flush with the fuselage: less turbulence, less error in sideslip • The Cessna has one, on the left side
  • Fast aircraft • Shock waves upset ports at high speed • So the probe goes ahead of the wing or fin tip

Why can the static source feel the wrong pressure?

Errors of the pressure head

  • Position error • The airflow round the aircraft changes the static felt: position error • At an angle to the air the flow speeds up past the port: it feels a little less than S
  • Manoeuvre-induced • In a pitch change the flow shifts: manoeuvre-induced error • Lasts about 3 s low down, up to 10 s at 30 000 ft: watch the values wander and settle
  • Instrument error • Instrument error: small faults from friction and manufacture, kept small by calibration • One correction card gives instrument + position error together
  • Lag and hysteresis • Lag: the needle reaches the right value a moment late • hysteresis: the reading going up differs from the reading coming down, because the capsule springs back a little differently

How are the pitot and static pipes joined on the Cessna 172?

The full system

  • Pitot: one tube • One pitot tube, under the left wing • Its line runs through the wing and down the door pillar to the ASI only
  • Static: one port • One flush static port, left side • Its line feeds the ASI, altimeter and VSI
  • Alternate static • Under the panel: the alternate static valve • Pull it if the static port is blocked
  • Before flight • On the ground a red pitot cover (with a REMOVE BEFORE FLIGHT streamer) keeps insects and rain out • Covers and plugs off and stowed; heater checked (warm in about 30 s)

The sketch: how does the ASI know your speed?

The S + D box

  • Draw a box and an oval • The case is the box, the capsule is the oval • Pitot (S + D) goes into the capsule, static (S) fills the case round it
  • Static cancels • S inside, S outside: they cancel • Only D moves the needle
  • Faster: more D • More speed → more D → oval swells → higher reading
  • The altimeter box • The altimeter capsule is sealed (nearly a vacuum), S fills the case • Climb: the case pressure falls, the capsule swells, the needle reads higher
  • Try it • Drag the height: the carriage rides up the column, the capsule and the needle follow

The pitot tube ices up. What does the ASI do?

Pitot blocked

  • Ice in the mouth • Pitot and drain blocked: the capsule keeps its old S + D • The air cannot come in and it cannot go out
  • Climb to 7 000 ft • Outside S falls; capsule still old • Difference grows → over-reads
  • Descend to 4 600 ft • Outside S rises; difference shrinks → under-reads
  • It acts like an altimeter • Level: frozen. Climb: up. Descent: down • Drag the height and watch
  • Drain hole open • The trapped air leaks out to static • The ASI falls to zero

The static vent blocks. What do the three instruments do?

Static blocked

  • Blocked at 5 000 ft • The case keeps its old S
  • ASI in a descent • Capsule gets the new, higher S + D; case still old • Difference grows → over-reads (dangerous: you are slower)
  • ASI in a climb • Under-reads: the opposite of a pitot block
  • Altimeter: frozen • It keeps showing 5 000 ft
  • VSI: zero • Capsule and case both stuck: no rate
  • Leaks • Pitot leak: ASI under-reads • Static leak, unpressurised: ASI and altimeter over-read slightly

Ice up a port and fly: what do all three instruments do?

Fly it blocked

  • Pitot iced at 5 000 ft • Ice plugs the pitot tube; the drain hole is iced too • Drag the height: the ASI climbs with you, ALT and VSI stay honest
  • Static iced at 5 000 ft • Ice over the static port: the case air is trapped • Drag the height: ALT frozen, VSI zero, ASI wrong

One port blocked, a sideslip, an emergency: what now?

Which instruments, what to do

  • Count the instruments • Pitot blocked: ASI, Machmeter • Static blocked: ASI, altimeter, VSI, Machmeter
  • Sideslip • The static port facing the air feels extra pressure → ASI and altimeter under-read • On the lee side it feels suction → they over-read
  • Static blocked: what to do • Select alternate static • No alternate? Last resort: break the VSI glass

Solve it: trace a blockage on the box

Solve it: a blockage

  • Solve it: trace a blockage on the box

Test yourself on chapter 20

Test yourself

  • Five questions • Answer each one, then read why