The Classic Boeing Airspeed Indicator

Honeywell SI-800 Airspeed/Mach Indicator

The Honeywell SI-800 Airspeed/Mach Indicator has been a standard instrument on the Boeing 737, 747, 757, and 767 for decades.

Boeing stopped installing the stand-alone indicator in favor of modern, reliable flat panel displays. Airlines are upgrading older cockpits with newer displays, so this old indicator will soon become a relic.

The instrument provides pilots with a surprising amount of information. Read on to find out how the classic SI-800 works and how pilots use it!

767 instrument panel with airspeed indicator highlighted.
Location of the Honeywell SI-800 airspeed indicator on a 757/767 panel

Inputs

All airspeed indicators need two air inputs. Ram air from a pitot tube and static (undisturbed) outside air from a static port, usually a hole on the side of the fuselage.

left side of 767 nose with two el shaped pitot tubes projecting out the side of the fuselage beneath the cockpit windows. Another image shows a static port on side of fuselage (6 inch diameter shiny aluminum circle with several small holes in the center. Warning placard above says: "Static port: Do not plug or deform holes indicated area must be smooth and clean")
Airspeed indicators need air from a pitot tube and static air from a static port

Small general aviation aircraft have airspeed indicators with air hoses connected directly to the pitot and static sources.

More advanced aircraft have Air Data Computers (ADC) that collect raw data from pitot tubes, static ports, and temperature sensors. The ADC processes the data and sends it to various aircraft systems including airspeed/Mach indicators (like the SI-800), altimeters, Flight Management Systems (FMS), Autopilot Flight Director System (AFDS), elevator feel computers, and more.

Pointers

VMO Pointer (Barber Pole)

At low altitudes, the VMO pointer indicates the maximum operating airspeed for the aircraft. Flying faster than VMO can cause structural damage. VMO on the 767 is between 340-360 knots (depending on aircraft serial number).

maximum speed VMO pointer from an airspeed indicator. Pointer is red and white striped and looks like a barber pole.
The VMO pointer or “Barber Pole”

Critical Mach Number (MMO)

Critical Mach Number (MMO) is the speed where air flow over the wing reaches (but does not exceed) Mach 1. Because airflow accelerates as it flows over the wing, MMO occurs before the aircraft reaches Mach 1. Critical Mach on a 767 is around 0.91 Mach.

When an aircraft exceeds MMO, a shock wave forms over the wing causing increased drag, buffeting, and possible loss of control. So it’s important to avoid flying subsonic aircraft above the Critical Mach Number.

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An interesting thing happens as an aircraft climbs to cruise altitude. The speed of sound decreases as outside temperature decreases. On long flights, the outside temperature can change dramatically at cruise altitude. This means that the Mach 1 (and MMO) airspeed is always changing.

The Air Data Computer constantly calculates the MMO airspeed. When Critical Mach Number drops below VMO, the “Barber Pole” turns into an MMO indicator. As the outside temperature changes, the Barber Pole moves to indicate the current Critical Mach airspeed.

In general (standard day, standard temperature decrease with altitude), the barber pole speed decreases as altitude increases.

Airspeed Pointer

The airspeed pointer shows the indicated airspeed in knots as generated by the Air Data Computer.

Image of airspeed indicator. speed numbers and tick marks around the outside similar to a clock. Speeds range from 60 at the top around clockwise to 400.

Top center of gauge is a window with digits that show current Mach Number - shows point 7 9 0.

Below center of gauge is a digital airspeed window that shows 2 8 0.

Two pointers on indicator: a speed pointer pointing to 290 knots. and a red and white V M O max speed pointer at 301 knots

Digital Data

Airspeed

The Indicated Airspeed (above 30 knots) is displayed in a digital format. This speed is identical to the speed depicted by the Airspeed Pointer. It’s nice to have an accurate digital display, especially on a bumpy approach.

Mach Indicator

The Mach window displays the Air Data Computer generated Mach Number from .400 to .999. The window is blank below .400 Mach.

Speed Bugs

Small pointers on the airspeed indicator are usually referred to as bugs. Pilots use the bugs as references for important takeoff and landing speeds. The Honeywell SI-800 has two types of bugs: Reference Airspeed Bugs and the Command Bug.

Reference Airspeed Bugs

The Reference bugs are small, white plastic pointers that snap onto a bezel around the indicator and are moved manually. They’re an elegant implementation of reliable low-tech.

Occasionally, a bug breaks or pops off the bezel (lost forever in the seat tracks). Maintenance techs usually place a spare bug on the bezel at the 12 o’clock position and can replace them when needed.

Animated GIF of my fingers moving the small plastic airspeed bugs on the bezel surrounding the indicator
The airspeed bugs slide on a bezel around the indicator.

The Command Bug

The orange airspeed bug behind the glass face of the indicator is the Command Bug. This bug is controlled by the Flight Management Computer (FMC) or manually with the IAS/MACH selector knob on the Mode Control Panel.

The autopilot/autothrottle system will use pitch and/or thrust to maintain the speed commanded by the Command Bug.

animated GIF split screen video: my fingers rotating a speed selector knob. And the command bug in the airspeed indicator moving as I rotate the knob to the desired speed.
Adjusting the Command Bug with the IAS/MACH selector knob

What do the bugs represent?

Pilots set the bugs before takeoff and prior to landing. The speeds are based on aircraft weight and performance (affected by runway and weather). Bug speeds are calculated by the Flight Management Computer, aircraft dispatcher, an aircraft performance service, or performance charts.

I use the Boeing 767 procedures at my company as reference for the following. Some terminology is modified for clarity. This is an overview and does not cover all situations. Procedures may differ on other aircraft and at other airlines.

If this information helps you when flying Microsoft Flight Sim, great! If you’re flying a real aircraft, you know better than to take my word for it – follow your company procedures.

Takeoff Bugs

  • Bug 1: V1 Decision speed – Take off should no longer be rejected above V1 (safer to fly than to stop).
  • Bug 2: VR Rotate Speed – Pilot begins to raise the nose.
  • Orange Command Bug: V2 – Minimum engine inop climb speed.
  • Bug 3: VRef Landing speed +40 – Takeoff flap maneuvering speed. Takeoff flap retraction begins after accelerating to this speed.
  • Bug 4: VRef Landing speed +80 – Flaps up maneuvering speed.
Takeoff Speed Bugs. White bugs positioned at 141, 148, 189 and 229 knots. Orange command bug at 154 knots.

Landing Bugs

  • Bug 1 and 2: VRef – Landing Reference/Threshold Crossing speed (bugs are positioned together).
  • Orange Command Bug: VApp Approach speed (VRef + additions for winds/gust).
  • Bug 3: VRef Landing Speed +40 – Flaps 5 maneuvering speed
  • Bug 4: VRef Landing Speed +80 – Flaps up maneuvering speed
Landing Speed Bugs: 2 white bugs positioned together at 141 knots. Other two bugs at 181 and 221 knots. Orange command bug is V Approach speed of 149 knots.

When Things Go Wrong

In the unlikely event of a problem, the indicator has four warning flags. If a flag appears due to an Air Data Computer malfunction, the crew can switch the Air Data source to bring the indicator back to life.

diagram of indicator with orange flags covering 4 windows in the center of indicator. Mach Flag and Airspeed flag indicate an air data computer failure. VMO flag indicates VMO pointer inop.  Command INOP flag indicates command bug is inop
Instrument illustration is similar to what is seen when aircraft is powered down.

The Future

It’s sad to see this old indicator go away, but the future is pretty bright. Newer flat panel displays provide pilots with similar airspeed data presented in a clear, intuitive format that improves situational awareness.

Animated GIF of indicator operating in cruise flight. speed needle is moving slightly within one or two knots of 280. Digital speed read out is moving between 279 and 280 knots. Mach display is flipping back and forth between decimal 7 8 8 and decimal 7 8 9
The classic SI-800 hard at work during cruise

18 Comments

  1. Yet another OUTSTANDING post on a subject I thought I knew something about! Your insights are always a privilege to read. Thank you again for taking the time to explain such a fascinating topic.

  2. Hello Ken,
    Great article as always. I need your help to understand this phrase :
    “In general (standard day, standard temperature decrease with altitude), the barber pole speed decreases as altitude increases”
    you also mentioned : “The speed of sound decreases as outside temperature decreases”

    so in a sense one should fly slower in colder weather because it might damage the aircraft? and what has speed of sound to do with that?

    Thank you again

    • We want to avoid flying faster than our Critical Mach Number (which is around 0.91 Mach, or 91% of the speed of sound).

      As temperature decreases, the speed of sound (Mach one) decreases. At high altitude, the barber pole points to the airspeed that equals 91% of the current speed of sound; our Critical Mach. When the speed of sound changes with temperature, the barber pole moves so we always know our Critical Mach speed.

      I hope that helps. Here’s a Wikipedia article that might also help:
      https://en.wikipedia.org/wiki/Critical_Mach_number

  3. Dear Captain Hoke

    Another fascinating article – you have the wonderful knack of covering potentially mundane topics in such a readable, intelligent and interesting way.

    I am a pilote manqué and live out my aviation dreams vicariously through you, including on FR24, of course. I sometimes wish you would post more items on your website but have discovered that one of the very few benefits of growing old and not remembering stuff so well is that I can re-read your old articles and it is as if they are brand new!

    Many thanks

  4. Great article Ken! I am wondering what has replaced this and what it looks like.

    Also I’m interested in knowing how the altimeter works in aviation. When landing at different airfields which are at different altitude themselves.

    • The replacement for individual instruments are integrated flat panel displays. There are pros and cons to both, but I like the flat panels a little better.

      Large Display Screen System

    • Scott,

      I didn’t address your second question. It’s going to be a future article. For now, when landing, our altimeters are set in reference to sea level. So, if the elevation of an airport is 650 feet above sea level, our altimeter will read 650′ on the ground. More on that later. 😀

  5. wow….. again you did it…. aero sav you really have great aviation skills and knowledge. thanks for
    your great articles, always well written and easy to understand. keep up the great work.
    happy clear skies.

  6. Found this update really interesting. Electronics have changed so much in the past 10 years. Have been following the Missionary Bush Pilot on his single engine aircraft. To me, a non pilot, well some hours on an old Fleet Canuck, it is interesting and confusing, Can’t imagine how complicated it is for you. Man you pilots have to be 100% on top of your game and sharp all the time.

  7. Thanks, Ken. Very educational. I am a low altitude GA pilot, and never gave a moment’s thought to altitude and temp effects on mach limits. Interesting topic.

    In thinking this through, does Mach 1 occur when air is no long compressible? Is that what is happening? So if it becomes, essentially, infinitely viscous, it suddenly offers much more resistance to flow, and if one tries to “push more air” onto a viscous boundary, say atop the airfoil when that area approaches M1, it can’t go anywhere, and this relatively incompressible fluid then squirts out wherever it can. Is the an explanation of the generation of shock waves?

    Thanks, -Mike Hodish

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