“Holy Micro’s Absolute AoA is a standout among GA angle of attack systems, partly for a straightforward install and rich feature set.”
Once limited to jets, angle of attack systems have become old hat for small GA airplanes. Properly installed (and calibrated), flying by AoA reference can mean better safety margins when landing on tight runways, not to mention less wear on tires and brakes. Additionally, the Federal Aviation Administration issued Special Airworthiness Information Bulletin 2024-07 to reinforce the benefits of installing AoA systems in hopes of reducing the risk for loss of control accidents. We certainly need help in that area, based on the wreck reports.
New York-based Holy Micro has been selling its $2,650 Absolute AoA system, which has a smart feature set and for professional technicians, a relatively uncomplicated installation with a critical calibration process. Here’s a summary.
Multi-sensor with LiDar
Holy
Micro said the Absolute AoA’s multiple sensors, including a flap
position scanner that uses LiDar (Light Detection and Ranging) on the
underwing-mounted probe, enables system accuracy regardless of flap
setting, density altitude, and aircraft loading.
The result is the Absolute AoA’s ability to serve triple duty for climb, cruise and landing where single-sensor systems primarily measure a wing stall. As for main hardware to install, there is the underwing probe with color-coded tubes that connect with an external CPU, a choice of IPS touch-capable panel displays (in-panel or top of glareshield), and an Android smartphone that the company calls the HMI (Human Machine Interface) that’s used for configuration and calibration.
Installation
In its most basic configuration, the company said it isn’t out of the question to install the hardware in an hour, though we think that’s a stretch for most, especially if connecting the system to an audio panel’s unmuted, unswitched audio input. These three audio wires are in the system’s seven-wire bundle. In alternate, the system can connect to a Bluetooth audio source (headsets are most common), and to an optional 5-watt external speaker.
The 6061-grade Aluminum underwing AoA must be mounted in a location where the airflow over the probe is relatively undisturbed by the aircraft. In general, this is to a flat area underneath the wing close to the leading edge (within 16 inches) and positioned within 60% of the wing’s chord length (excluding the wing flap or aileron). The most preferred location is out of the prop wash. Last, a good option is mounting the probe to an existing inspection plate using a doubler plate.
In order for the flap position detection feature to work, the probe must be installed in front of the flap or use the flap scanner pod. The AoA probe is designed with four pressure ports: two that capture ram pressure from the airflow, and the other two ports (angled 45 degrees aft) capture the static pressure. The pressures are transmitted via hoses to the sensor unit mounted in the wing near the probe secured using a bracket included in the kit.
The sensor converts the pressure data into digital signals that are communicated to the central processing unit, an ABS plastic device that can be installed behind the instrument panel. The CPU communicates with (and provides power to) the AoA display, plus it generates two types of audio alerts (pilot selectable) that are played over the headphones or through the external speaker. The system uses a 1-amp circuit breaker and total draw is less than 3 watts.
Calibration
As with any angle of attack system, this is perhaps the most critical portion of the installation. The Absolute AoA’s main software runs within the CPU, which broadcasts a Wi-Fi signal to connect with the HMI or via a wired RS-232 serial connection. Before flying it, the first pieces of data to enter in the HMI and serving as the baseline for calibration is the aircraft’s performance specifics (sourced from the aircraft’s POH and weight and balance data). The data then generates the Carson speed – the reference speed that is the optimum balance between fuel efficiency and speed. It’s roughly 32% above the best lift over drag speed and this number serves as the baseline for calibrating the system.
The in-flight calibration consists of first climbing to the altitude called for by the HMI after pressing the “AoA Calibration in the Air” button on the HMI. Once at the level altitude, the system does a barometric calibration, followed by an indicated airspeed calibration where you slow to the Carson speed for the airplane and click “calibrate.” The HMI will display the IAS measured by the AoA system, which should match the IAS on the airplane’s airspeed indicator.
Owners and installers we spoke with said it could take several flights to dial in the right calibration. Find smooth air and bring a safety pilot.
Flying with it
The pilot has the option of choosing from different AoA presentation styles: chevron, bar, sweep needle, and a performance mode. You can change the presentation mode by touching the AoA Presentation Option key. Perhaps the most common and familiar to those who have flown with AoA systems will be the chevron presentation, which has 13 segments. Each segment corresponds to a specific angle of attack and of these 13 segments, the main four segments correspond to the Carson cruise speed, best glide speed, best approach speed, and the stall.
Each display mode is intuitive and there is a lot of data. As one of many examples, the angle of attack for Carson cruise is displayed by the single green bar, and the yellow triangle displays the angle of attack for best glide. In between these two segments is another that corresponds to an angle of attack that falls in between these two values. If audio tones are enabled, a 400Hz intermittent tone begins at the best glide speed. Its pulse rate increases as you approach best approach speed.
The full blue donut corresponds to the angle of attack for best approach, where the intermittent tone transitions into a solid 400Hz. Between the angle of attack for best glide and best approach, there are four segments that correspond to angles of attack values in between. Beyond best approach, a 1600Hz intermittent tone starts and pulse rate increases as you near aircraft’s own stall warning. The inverted red triangle corresponds to the angle of attack for stall.
Between the angle of attack for best approach speed and stall, there are four segments that correspond to angles of attack values in between. If announcement is enabled, “getting slow” plays immediately after best approach speed, transitioning to a “too slow” callout as the aircraft gets closer to the stall angle that was initially calibrated.
The performance mode presentation also has 19 segments but presents angle of attack in a slightly different way. The donut in the middle of the display is the reference point with two vertical tapes with color-coded angle of attack on the right and indicated airspeed on the left. Unlike the other modes, this performance presentation is the only one that allows the display of user-set reference points, including Vx and Vy.
On the panel-mounted display, there is an additional mode of AoA indication that’s not available on the glareshield display: a sweep needle. It has 13 segments presented in a curve on the left side of the display and a needle hinged on the right side moving to point at the appropriate location on the curve.
When given the option, in the world of AoA indicators it’s common for many buyers to choose the glareshield mount because that places the display at a spot where the eyes are already looking during approach and landing – outside the windshield.
NORSEE approval
In the end, it’s up to the installer to determine how the Holy Micro AoA system will be signed off and depending on the installation of the components and especially the sensor probe. In some cases, it’s possible the system might be considered a major alteration. But structural modification aside, AoA systems are intended for supplemental use only, and they shouldn’t be used to replace the aircraft’s flight instruments or stall warning system.
Plus, remember the intention of the FAA’s NORSEE (Non-Required Safety Enhancing Equipment) basis for approval. This is for equipment that’s intended to enhance safety and is considered complementary and secondary (nonessential) to the required equipment.
Last, it’s worth mentioning that Holy Micro has extremely detailed and well-written installation documentation, plus good field support. In the world of aftermarket GA AoA systems, Absolute AoA is a standout. Learn more at holymicro.com.