CDI and HSI indications

Keep course, heading, and track distinct

The selected course defines the desired line and direction. Heading is the direction the nose points. Track is the actual direction of movement over the ground. In no wind, heading and track may match; in a crosswind, a heading into the wind can maintain the selected track. A centered CDI can therefore coexist with a heading different from the course.

Example: the desired eastbound course is 090°. In an invented steady wind from the north, suppose heading 082° maintains track 090° with a centered needle. The 8° difference is a wind-correction angle for these assumed conditions, not a universal setting. Turning to 090° merely to align heading and course would allow drift south.

A course-deviation needle shows displacement from a line, not where the nose points. A bearing pointer, when installed and selected to a valid source, points toward a station or waypoint; it is a different indication from a course arrow. A heading bug is a selected heading reference and may command an autopilot in heading mode. None of those symbols should be silently treated as another.

IFH 5-23–5-24: HSI and heading-bug functions · IFH 9-14: tracking with wind

Read a conventional CDI in the selected-course frame

A conventional CDI separates the OBS course setting from the heading indicator. For VOR, set the intended course and verify TO/FROM. When you mentally face in that course direction, the needle indicates which side the selected line lies on. A centered needle means on the line, subject to valid guidance; a deflected needle indicates course displacement. The needle does not tell you to use a bank equal to the number of dots.

At a fixed position with unchanged VOR course selection, changing heading does not change CDI deviation or TO/FROM. For example, on the 090° radial, 270° TO remains centered even while the airplane points north. If you select 090° FROM but intend to fly inbound west, a conventional display’s left/right sense is reversed relative to that travel. Select the course you intend to follow and orient before correcting.

When an intercept heading is far from the course, the needle’s direction is still referenced to the course, not necessarily to the airplane’s present left or right wing. Draw the course or use the HSI geometry to decide which heading will cross toward it. “Always turn toward the needle” is incomplete: source, selected course, direction of travel, and validity must already be established.

IFH 9-11–9-14: CDI, TO/FROM and heading independence

Use the HSI picture without skipping its parts

An HSI combines heading and course-deviation information. The compass card usually rotates so current heading is under the top index. The course arrow shows the selected course on that card; its displaced center bar shows the selected course line relative to the aircraft symbol. TO/FROM, source labels, flags, and sometimes bearing pointers add information. Electronic implementations differ, so learn the installed symbols and controls.

Imagine heading 090° and selected VOR course 090° FROM. The course arrow is toward the top, and a bar left of the aircraft means the course is left. Turn the nose while keeping the same ground position: the card and course arrow rotate, and the bar’s arrangement rotates with the selected course. The underlying VOR deviation and TO/FROM relationship do not change merely because the display turned.

This geometric picture reduces the mental work of intercepts and some reciprocal-course situations. It does not prove that the selected source is the intended one, that a course is authorized, or that the aircraft’s heading information is accurate. An HSI can present valid navigation on an erroneous heading card, or an accurate heading card beside invalid navigation. Cross-check both functions.

IFH 5-23–5-24: HSI components and interpretation · IFH 9-12–9-13: HSI VOR presentation

Verify what is driving the needle

A single electronic HSI can display VOR/LOC from NAV1 or NAV2, or a GPS/RNAV leg. Source names and colors are installation-specific; do not use color alone to establish the source. Confirm the label, active frequency or waypoint, selected course, validity, and displayed sensitivity. A standby frequency or a route merely loaded in a navigator may not be the source currently displayed.

For GPS, verify the active from/to waypoint leg, desired track, and sequencing state. “Direct to” creates a path from the present position toward a waypoint; it may not be the cleared course to that waypoint. In many installations OBS or suspend mode stops normal sequencing or changes the course-selection function. The same knob that selects a VOR course may not alter an ordinary GPS flight-plan leg. Use the approved navigator and display guidance for that installation.

Automation needs a separate check. A navigation source can be valid while the autopilot remains in heading mode. NAV armed commonly means waiting for capture, not already tracking. After a source change, verify the displayed source and actual armed/active flight-guidance modes, then check the airplane’s response. A coupled autopilot can follow the wrong valid leg very accurately.

AIM 1-1-17: GPS equipment, training and operating requirements · AIM 1-1-18: WAAS functions and equipment-dependent operation · ACS II.B, printed page 7: navigation modes and failures

A dot changes meaning with the source and mode

VOR deviation is angular relative to the station. On a typical five-interval display, one interval is about 2° and full scale is about 10°. At 5 NM, a small angular error is a smaller lateral displacement than the same error at 30 NM. Increasing needle movement near the station can result from geometry rather than a suddenly stronger wind.

Localizer guidance follows a fixed transmitted course. Its full width from full-scale fly-left to full-scale fly-right is adjusted to about 700 feet at the runway threshold. It is substantially more sensitive than VOR near final approach. Turning the OBS does not move the transmitted localizer line. The HSI course setting still helps orientation and may matter to the flight-guidance system.

GPS/RNAV displays commonly use distance-based cross-track scaling that changes with phase and operation. As one historical/basic GPS example, en route, terminal, and nonprecision approach full-scale values can be ±5 NM, ±1 NM, and ±0.3 NM. These values are not a rule for every navigator or every approach. WAAS LP/LPV and other guidance can include angular-like scaling that narrows toward the runway. Read the actual mode and scale annunciation rather than assuming one dot always equals one mile or two degrees.

For an explicit arithmetic example, if the actual display states full scale ±1 NM and divides each side into five equal intervals, one interval represents 0.2 NM. If it later states full scale ±0.3 NM with the same intervals, one interval represents 0.06 NM. Those computations explain why an unchanged lateral error can look much larger after a scale transition; they do not prescribe corrections for a particular aircraft.

IFH 9-11: VOR angular scale, read with errata · IFH errata: correct VOR full scale to 10° · AIM 1-1-9: localizer course width · AIM 1-1-17: basic GPS phase-of-flight scaling · AIM 1-1-18: WAAS scaling and service levels

Localizer direction requires its own check

For the localizer front course inbound, ordinary CDI sensing indicates the course to the needle side. On the back course inbound, or front course outbound, a conventional display without reverse-sensing capability normally requires correction opposite the needle’s deflection. The OBS setting on a conventional CDI does not reverse the received localizer sensing.

A correctly configured HSI can provide the appropriate geometric sense when its course arrow is set to the front course. Approved back-course or reverse-sensing modes may also change the flight-guidance response. Confirm the actual display/autopilot instructions; do not infer the required mode from a familiar VOR habit. Do not fly a localizer back-course approach unless a procedure is published and it is authorized by ATC.

This lesson teaches lateral-display recognition, not an ILS or back-course approach procedure. Later lessons add protected segments, intercept altitudes, vertical guidance, minima, and missed approaches. A centered lateral needle says nothing by itself about a safe altitude or a valid glidepath.

AIM 1-1-9: front/back course sensing and authorized back-course use · IFH 5-23–5-24: HSI localizer interpretation

An attractive picture can contain invalid data

Inspect source-invalid flags, loss-of-integrity messages, heading warnings, and any failed or removed indications. A GPS position may still be calculated when integrity monitoring is unavailable; that does not authorize continued use for an operation requiring assured integrity. A map symbol and GPS CDI fed by the same receiver are not independent confirmation. Two screens using the same heading source can also repeat one error.

If the indications do not fit the clearance and expected position, maintain control, verify selections and modes, and compare independent sources that are appropriate and usable. Do not make a large turn just to center a doubtful needle. For example, a VOR FROM indication where a verified position calls for TO may mean the reciprocal was selected, the wrong station/source is active, or the position assumption is wrong. Resolve the discrepancy before committing to a turn.

Keep altitude, airspeed, attitude, and heading in the cross-check while interpreting navigation. Near capture, a rapidly moving needle can attract all your attention. Brief source changes and divide programming into manageable segments. If the workload or equipment does not support the clearance, request clarification, more time, or a suitable revised clearance. Follow AFM/POH and approved supplements for actual failure or reversion actions.

AIM 1-1-17: integrity alerts and GPS operating restrictions · AIM 1-2-4: GPS interference and correlated indications · ACS V.A, printed page 12

Read a complete cockpit state

Training example: an identified valid VOR is selected, the intended outbound course is 090°, the HSI course arrow is 090° FROM, and the CDI is centered. Heading is 082° and independently observed ground track is 090°. The approved equipment is operating normally; the numbers are invented study conditions.

1. Path: you are on the 090° radial and following its outbound course.
2. Control: the difference between heading and track is consistent with a wind correction; centering the needle does not require heading 090°.
3. Change: the pilot selects GPS but the active leg goes direct to a different waypoint. A centered GPS needle now describes that different path, not the previously assigned VOR course.
4. Verification: compare the active leg and displayed source with the clearance; restore or program the appropriate authorized path and verify guidance modes before following it.

As an additional written exercise, cover the source label and ask whether a half-scale needle tells you an angular error or a distance error. It cannot answer that question alone. Uncover the source, mode and scale, then explain the indication in words before choosing a correction. The navigation trainer can extend this comparison, but the table, examples and reasoning remain usable without JavaScript.

IFH 9-12–9-14: course and heading relationships · AIM 1-1-17: correct use of navigator functions · ACS V.A, printed page 12