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!
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.
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.
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).
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.
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.
The airspeed pointer shows the indicated airspeed in knots as generated by the Air Data Computer.
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.
The Mach window displays the Air Data Computer generated Mach Number from .400 to .999. The window is blank below .400 Mach.
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.
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.
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.
- 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.
- 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
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.
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.