Navigation & Instruments

ATA Chapter 34 — N720AK Systems Reference

Overview

N720AK’s navigation and instrument suite is built around the Dynon Skyview HDX EFIS as the primary flight display, with the Garmin GTN 650 providing certified IFR GPS/Nav/Com capability. The pitot-static system uses a Dynon heated pitot with integrated AoA probe.

Components

How It Works

Dynon Skyview HDX

The Skyview HDX provides:

• Primary Flight Display (PFD) — attitude, airspeed, altitude, heading, VSI
• Multi-Function Display (MFD) — moving map with terrain
• Engine monitoring — all EGT, CHT, oil, fuel flow, fuel pressure
• Traffic display (ADS-B In, dual-band — see below)
• Autopilot interface
• Checklists

Dynon Equipment Serial Numbers (registered 2017-05-28):

GPS Position Sources — Who Draws the Map

As of the 2026-08-19 config (SkyView 17.6), the Dynon SV-GPS is POS 1 — promoted from POS 2, making it the primary position source for the moving map and synthetic vision at its native 5 Hz, per Dynon’s recommended arrangement. The GTN 650 (via SV-ARINC-429 + Aviation-format serial) remains GPS 1, the fallback position source at 1 Hz.

Flight-log evidence supports the switch. With the GTN as position source (July 2026 logs, ~13 flights), the logged position stream showed ~9–14 kt median discrepancy between position-implied and reported ground speed with occasional 2–3 s position freezes — consistent with jumpy synthetic-vision rendering — and the log’s GPS Fix Quality / satellite-count columns were unpopulated in flight (the Aviation-format feed does not carry them). With the SV-GPS as POS 1 (2026-08-19/20 logs), every in-flight sample showed SBAS-grade fix quality (2), 12 satellites, sub-knot median position jitter, zero teleports, and immediate fix at startup. Check the live source under SETUP MENU > LOCAL DISPLAY SETUP > GPS FIX STATUS.

ADS-B — What You Transmit vs. What You Receive

Two different boxes on two different links. The distinction determines what traffic you actually see and what you file.

The dual-band receive is the part that matters in practice: on the 978 link alone, traffic only appears where a ground station is rebroadcasting it. The 1090 side sees 1090ES-equipped aircraft directly — which is the difference between having a traffic picture in the mountains west of Boulder and not having one.

Note the asymmetry: ADS-B Out is 1090-only, ADS-B In is dual-band. That combination has no clean ICAO surveillance code for the receive side; see the filing note below.

ADS-B and Mode S Filing Data

With no AFMS (experimental), the PBN declaration rests on Garmin’s published PBN capabilities document plus the TSO-C146c install; AC 90-100A allows Part 91 operators to self-qualify. Oceanic codes (A1, L1) require dual GPS plus an LOA — not filed. RNP AR (T1/T2) is beyond the GTN 650’s hardware — never filed.

Remarks are mandatory, not optional. Airworthiness certificate limitation 7: “When filing a flight plan, the experimental nature of this aircraft must be listed in the remarks section.” File RMK/EXPERIMENTAL.

Nearest Airport Emergency: Holding the NEAREST button on the Dynon activates the autopilot to fly directly to the nearest airport matching the current filter settings and automatically tunes the radio to that airport’s frequency. See the Dynon HDX Pilot’s Guide for filter configuration and exact behavior.

Garmin GTN 650

The GTN 650 provides:

• IFR-certified GPS approaches (LPV, LNAV/VNAV, LNAV)
• VOR/ILS capability via single Bob Archer nav antenna
• Com radio
• Flight plan management

SD Card Formatting: The GTN 650 SD card must be formatted using the SD Card Formatter utility. Formatting with macOS Disk Utility or Windows (including Parallels) does NOT work. (VAF thread)

Garmin GTN 650 Database Update

1. On a Windows machine, open Garmin Aviation Database Manager and download the current navigation database
2. Write the database to an SD card formatted with the SD Card Formatter utility (do NOT use macOS Disk Utility or Windows format)
3. Insert the SD card into the GTN 650
4. Power on — the unit should prompt to load the database on startup
5. If the database shows as a “future” database and does not load automatically, hold down the right knob click button during startup to force-load the database

GTN 650 ARINC 429 configuration

As set on the aircraft (Sam, 2026-09-23), read off the GTN’s ARINC 429 config page:

SDI: Common. The other options offered are LNAV 1 and LNAV 2.

IN 1 is the GTN receiving from the SkyView; OUT 1 is the GTN transmitting to it, through the SV-ARINC-429 module. SDI (Source/Destination Identifier) tags transmitted labels with a nav-source number so a receiver can tell two navigators apart — Common is the correct setting for a single-navigator installation like this one, and means the SkyView accepts the GTN’s labels without needing them tagged as LNAV 1 or LNAV 2.

What the SkyView does and does not get from the GTN. Over the SV-ARINC-429 the SkyView receives lateral CDI with auto-scaling, GPSS roll steering, and vertical deviation for approach glideslopes (ILS) and GPS glidepaths (LPV, LNAV/VNAV, +V). It does not receive the GTN’s enroute VNAV (VPATH) descent guidance: the GTN computes that path on its own screen (TOD marker, VS Required, “Vertical Track” message), but the SkyView’s Glideslope/VNAV indicator shows only an ILS glideslope, a GPS approach glidepath, or SkyView’s own VFR VNAV to a runway, and the Dynon autopilot’s VNAV mode couples only to those. A GTN VNAV descent is therefore flown in VS mode against the GTN’s VS Required with the altitude bug on the bottom altitude. (SkyView HDX Pilot’s User Guide Rev T §4 and §8; Dynon forum, staff reply 2018-09-06 on ARINC 429 VPATH.)

Garmin GTN 650 Maintenance (ICA)

The GTN 6XX/7XX Part 23 AML STC Maintenance Manual (190-01007-A1 Rev 4) carries Garmin’s Instructions for Continued Airworthiness. The STC itself does not apply to an experimental, but the ICA is the only manufacturer-published maintenance schedule for the unit. There are no airworthiness limitations; the unit runs a self-test at power-up and continuous built-in test, and surfaces failures as system messages or red-X fields. Intervals are tracked in recurring-items.tsv.

Troubleshooting notes from Chapter 5 of the same manual that matter in the cockpit:

• No position after power-up (“SEARCHING SKY”): a cold almanac download can take about 20 minutes. Get clear of hangars, buildings, and trees first. Not a failure by itself.
• GPS signal drops when the COM transmits or avionics come on: 1575.42 MHz interference from the VHF COM, or ELT antenna re-radiation. Fixes are antenna separation, a notch filter in the COM coax, or disconnecting the ELT coax to test.
• Unit boots into configuration mode: a software loader card is in the SD slot. Remove it and insert the database card.
• “COM RADIO – COM locked to 121.5 MHz”: the COM remote-transfer input was held for about 2 s. On N720AK that input is the stick toggle UP (COM1 standby↔active swap; see sys-27). Lockout tunes 121.5 active and freezes tuning; hold the toggle UP about 2 s again to exit. The same gesture, or holding the GTN volume knob about 2 s, is the Pilot’s Guide §3.3.4 emergency-frequency shortcut and works even with the display dark.
• “COOLING – GTN overtemp”: the backlight auto-dims; check fan and airflow. “COOLING FAN – failed” is the fan itself (it does not run while the unit is cool).
• “LOSS OF INTEGRITY (LOI)” and “GPS NAVIGATION LOST” are the two GPS-side messages: LOI means verify position with another source; NAVIGATION LOST (insufficient satellites, or RAIM found an erroneous position) means switch to another source.
• Database SD card: Garmin’s database card associates with the first GTN it is inserted into and will not work in another unit.
• Power-up self-test values (Instrument Panel Self-Test page, not a required check): CDI half-scale left with TO flag, glideslope half-scale up, OBS/desired track 149.5°, all annunciators on, distance 10.0 NM, time 4 min, bearing 135°, groundspeed 150 kt, waypoint GARMN, GPS integrity invalid.

TAWS-B (GTN 650)

The GTN 650 carries TAWS-B, not the base Terrain Proximity. The distinction matters: Terrain Proximity is a display — shaded terrain with no alerting logic and no aurals. TAWS-B adds FLTA, Premature Descent, Excessive Descent Rate, Negative Climb Rate and the 500 ft callout.

Not required equipment. 14 CFR 91.223 applies to turbine-powered U.S.-registered airplanes with six or more passenger seats excluding pilot seats. A four-seat piston experimental is outside it, so TAWS here is elective — and nobody else has written its procedures.

What it needs

Only two things: a valid terrain/obstacle database, and a valid 3-D GPS position solution. No radar altimeter — that belongs to Class A TAWS. Height above terrain is GPS altitude against the database.

TAWS runs on GSL altitude (GPS altitude converted to MSL), not baro. GSL needs no altimeter setting and is immune to pressure and temperature error, so on a cold day or with a stale setting the altimeter drifts and TAWS does not. When they disagree, that is not evidence TAWS is wrong.

The audio path — and where it breaks

The GTN’s alert audio (P1001-4/23) reaches the GMA 245 Alert 1 input (J1-31/32), the same shielded pair that carries approach callouts. Every TAWS aural comes through the audio panel; there is no separate annunciator.

Two consequences specific to this airplane:

1. Lose the GMA 245 and you lose the TAWS voice while keeping the radio. The audio panel’s failsafe hard-connects COM1 from the GTN straight to the headphones — COM1 only. Alert 1 is not in that path. You would be flying a terrain system whose alerts you cannot hear. Same shape as the CO Guardian alarm that sounds in the cabin but not in the headsets.
2. Lose the main bus and TAWS goes entirely. The GTN’s COM/NAV power relays are held closed by AV MSTR from a main-bus feed, so a main-bus loss darkens the GTN even with its essential-bus breakers hot — and takes the GMA 245 with it. The electrical emergency where you might be descending toward terrain is precisely the one with no terrain alerting. See Electrical Power — the relay-coil re-source is on the annual fix list.

Useful corollary: the power-up TAWS self-test exercises that whole audio chain. Hearing the test aural on the ramp proves GTN alert output → shielded pair → Alert 1 → headset. Self-test is disabled above 30 knots groundspeed, so it is a ramp item.

Alerting, in brief

Red PULL UP is the only warning-level annunciation and covers four alerts. Yellow is TERRAIN, OBSTCL or WIRE — and four different alerts all show yellow TERRAIN (FLTA terrain caution, PDA, EDR caution, NCR), so the annunciation cannot tell you which one fired and the voice has to.

FLTA minimum clearance, level flight / descending: en route 700/500, terminal 350/300, approach 150/100, departure 100/100. FLTA auto-inhibits below 200 ft AGL within 0.5 NM of the approach runway, or below 125 ft AGL within 1.0 NM of the threshold — it stops protecting you where landing accidents happen.

Manual TAWS Inhibit (terrain page → Menu) disables only PDA and FLTA. EDR, NCR and the 500 ft callout stay live, and the inhibit does not survive a power cycle.

Full alert reference: GTN 650 Pilot’s Guide §10.4.

Pitot-Static System

The pitot tube incorporates a second orifice angled to measure differential pressure for Angle of Attack (AoA) display on the EFIS. Pitot heat is activated by the PITOT HEAT switch.

The Pitot/AoA Probe Is the Single Point of Failure

The redundancy is in the computers, not in the probe. Static is well covered; pitot and AoA are not covered at all.

The Dynon AoA/Pitot probe carries pitot and AoA ports only, no static port — static comes from the two aft-fuselage ports independently. So the exposure is precisely the pitot/AoA probe under the left wing, and altitude and VSI survive its loss.

Three consequences worth understanding:

1. A pitot blockage produces agreeing wrong data. Both SV-ADAHRS units breathe through the same probe, so a blocked or iced pitot gives them identical bad airspeed. They agree, so no ADAHRS CROSS CHK ERROR is raised. The cross-check protects against a unit failing, not against a shared source being wrong — which is the more likely failure.

2. AoA is not a backup for unreliable airspeed, and stall warning goes with it. OnSpeed uses the Dynon probe rather than independent sensors, so a pitot blockage takes airspeed, Dynon AoA, and OnSpeed simultaneously. Dynon stall warning is AoA-derived, so it goes too.

3. The attitude solution itself depends on airspeed. The SkyView installation guide is explicit: “SkyView’s attitude calculation requires airspeed from pitot and static. A GPS source can be used as a backup if the pitot and/or static source fails, but should not be the primary source.” So dual ADAHRS does not make attitude immune to a probe blockage — what protects attitude is GPS, not the second ADAHRS.

What remains after a pitot blockage: attitude on GPS backup, altitude and VSI on the independent static system, pitch attitude, power setting, and GPS groundspeed — with no low-speed protection.

Dynon’s own guidance assumes shared air lines: the install guide says to “consider ‘teeing’ off of existing lines.” Each module does have its own 1/8“ NPT AoA/pitot/static ports, so a genuinely independent second air source is physically possible with a second probe — but Dynon neither describes nor recommends it, and a single pitot is the norm across certified light IFR aircraft.

Cross-reference: OnSpeed AoA for the shared-probe detail.

Static blockage is the mitigated case: use the alternate static valve. Pitot blockage has no mitigation but pitot heat and preflight discipline.

Cross-reference: OnSpeed AoA for the shared-probe detail.

Inspection & Maintenance

Alternate Static Valve

The alternate static source valve is a toggle on the upper left panel. Reference: Steinair pitot-static toggle switch

Right Wing Com Antenna Access

To install or remove the right wing com antenna, the outboard aileron push-pull tube must be removed first.

91.411 / 91.413 Inspections

Per 14 CFR 91.411 and 91.413, the altimeter system and transponder must be inspected every 24 calendar months if operating in controlled airspace requiring this equipment (IFR flight, Class B/C airspace).

ELT — Artex ELT 345

• Beacon ID: 2DC88 5940E FFBFF
• Registration: NOAA SARSAT, registered 2025-11-18, expires 2027-11-18
• Frequency: 406 MHz
• Registration portal: beaconregistration.noaa.gov
• Registration form PDF saved in GDrive Private/Registration/ELT_registration_form.pdf

Battery: Artex P/N 8322 (replacement ordered via Aircraft Spruce, order #17716532, 2025-11-12)

References

• Garmin GTN 6XX/7XX Part 23 AML STC Maintenance Manual (190-01007-A1 Rev 4) — ICA: inspection intervals, bonding check, troubleshooting and system-message tables, self-test values, configuration-mode overview, database summary.
• Dynon SkyView HDX Pilot’s Guide (Rev T) — SkyView 17.6
• Dynon SkyView HDX Pilot’s Guide (Rev Q)
• Dynon SkyView EMS Gauge Customization
• Dynon SkyView Third-Party Device Connection (Rev E)
• TLAR Pilot Guide (v7.80)
• Pitot/Static/AOA Air Kit Installation Guide (Rev 04) — Quick-disconnect plumbing kit. Variants: A=SkyView ADAHRS, B=EFIS+backups, C=Steam. Tubing: 25’ green (pitot), 32’ white (static), 23’ blue (AOA), all 1/4“.
• Gretz Aero Pitot Tube Mounting Bracket Installation — Left wing bottom, outboard of main spar inspection plate.
• Dynon AOA/Pitot Probe Installation Guide (Rev C) — P/N 100141-000 (unheated), P/N 100667-000 (heated). Doc 100740-001.