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OnSpeed AoA

ATA Chapter 34 — N720AK Systems Reference

Overview

The OnSpeed system is an audio angle-of-attack (AoA) indicator that provides continuous tone-based feedback on the aircraft’s energy state. It uses differential pressure from the pitot-static system to compute AoA and delivers audio tones through the intercom.

OnSpeed shares the Dynon heated AoA/pitot probe — it has no independent air data source. So OnSpeed is not a redundant AoA reference. A blocked or iced pitot takes airspeed, Dynon AoA, OnSpeed, and the AoA-derived stall warning at the same time, and because both SV-ADAHRS units breathe through the same probe they will agree on the bad data and raise no cross-check error. See The Pitot/AoA Probe Is the Single Point of Failure.

The practical upshot: the tones are a stall-margin aid, not a redundant airspeed system. In an unreliable-airspeed event they are gone, and the remaining references are pitch attitude, power setting, and GPS groundspeed. Altitude and VSI survive, because static comes from the two aft-fuselage ports rather than the probe.

Components

ComponentPart NumberSupplierNotes
OnSpeed boxFlyOnSpeedMain processor
Pressure sensorsShares the Dynon heated AoA/pitot probeDynonNo independent sensors. Common-mode failure with airspeed and Dynon AoA.

How It Works

OnSpeed converts AoA into a continuous audio cue. As the wing approaches stall, AoA rises and the tone changes to give the pilot a head-out-of-the-cockpit indication of energy state and stall margin. The tones are the primary reference; the percent indicator on the panel is descriptive after-the-fact information, not directive.

Tones — What They Mean

As you slow down (AoA increases), you progress through five regions:

 FAST ─────── L/Dmax ────── ONSPEED ────── SLOW ─────── STALL
 (silence)   (low-pitch    (ONSPEED       (high-pitch   (stall warning
              pulsing)      solid tone)     pulsing)      buzz)
RegionToneMeaningPilot Action
FastSilenceAbove best-glide. Positive energy margin.No action.
L/D~MAX~ → ONSPEEDLow-pitch pulsing (slow → fast)Decelerating into the approach range. Start of low-pitch tone ≈ V~Y~ / best glide.Normal deceleration.
ONSPEEDSolid 400 Hz toneBalanced effective power — V~X~ / V~REF~ / max sustained turn rate.Hold this.
Below ONSPEEDHigh-pitch pulsing (slow → fast)Energy deficit — unsustainable.Push: throttle, nose down, or both.
Stall warningHigh-pitch buzz (20 pps)At the aerodynamic limit.Unload — reduce AoA immediately.

The Push / Pull Decision

The pattern alone tells you the action — no airspeed lookup required:

  • High-pitch pulsing → push (throttle forward, nose down, or both)
  • ONSPEED solid tone → hold; you are balanced
  • Low-pitch pulsing → pull (throttle back, allow pitch to increase)
  • Stall warning buzz → unload immediately

Logic is the same in level flight, in a turn, climbing, or descending — AoA-based, so the tones automatically account for bank, weight, and load factor.

Mapping to V-Speeds

For N720AK, the OnSpeed tone progression maps approximately to the published V-speeds:

ToneApproximate condition
Start of low-pitch toneV~Y~ / best glide (~95 KIAS clean)
ONSPEED solid toneV~X~ / V~REF~ approach (~80 KIAS clean)
Stall warning buzzApproaching V~S~ for the configuration

Speeds vary with weight, bank, and configuration — fly the tone, not the airspeed.

Audio Routing

OnSpeed audio is delivered to the headsets via the Dynon PFD audio output, mixed through the GMA 245 audio panel.

Muted Mode

If the pilot mutes audio:

  • All tones go silent except the stall-warning buzz, which overrides mute as a safety feature.
  • Stall warning in mute fires only if AoA > stall threshold AND IAS > the mute-under-IAS setting (default 25 KIAS).

Calibration

Wiring

OnSpeed connects to the Dynon PFD for power, data, and audio output.

OnSpeed Connector Pinout

Power / Ground

PinFunctionWire Color
112V power (from PFD, 2A fuse)Red
4Ship groundBlack

Data Inputs

PinFunctionWire Color
21Flap pot wiperWhite
25EFIS serial4 TX lineBlue

Button / Lower Console

PinFunctionWire Color
5Ground (to “−” on button, “C1” on button, volume pot “−/CCW”, pilot lo)Black
2Volume pot CWRed/white
9Volume pot wiperOrange/white
11“+” on buttonOrange/blue
23“NO1” on buttonBrown/white

Control cable wiring (6-conductor, button to OnSpeed box):

WireColor
1Red/white
2Orange/white
3(none)
4Orange/blue
5Brown/white
6Black

Headset Audio Output

PinFunctionWire Color
10Pilot audio rightPurple/green
22Pilot audio leftPurple/yellow

Emergency Maneuvering — Why the Turnback Has No Altitude Number

FlyONSPEED’s engine-out guidance is one line: “maintain ONSPEED and fly the airplane to the crash.” It holds from the failure through the flare.

The reason it replaces a number rather than supplementing one: ONSPEED is the AoA that gives best sustained turn rate and smallest sustained turn radius, and it is the same AoA at any weight, density altitude or G. A briefed turnback altitude is a proxy that is only valid at one weight, one wind, one lateral offset and one runway — which is a different set on every takeoff. FlyONSPEED declines to publish a decision altitude at all: it is “airplane dependent” and “can only be determined by practice.”

So N720AK’s Turnback Procedure is TLAR at ONSPEED, and the 200 ft AGL line in it is a floor — where the turn definitely fails, not where it works.

Two details from the same source worth knowing:

  • If there is any doubt about the energy to reach the runway, turn into the wind and slow to ONSPEED. That is the maneuver, not a fallback.
  • FlyONSPEED calls for “lift” flaps in the turn-back — takeoff setting, else half or less. N720AK’s checklist does not mention flaps in the turn.

Inspection & Maintenance

References