LDA vs. SDF Approaches: Offset vs. Width, Course Accuracy, the 30° Rule, Which One Can Have a Glideslope, and How to Fly Each
If you fly into smaller airports or train in older instrument environments, you’ll eventually see a plate labeled LDA RWY XX or SDF RWY XX. Both look “localizer-ish” on the needle, and pilots routinely lump them together as “the offset ones.” That’s where the confusion starts — because the defining feature of the LDA is that it’s offset, while the defining feature of the SDF is that it’s wide. They are different facilities, built for different reasons, with different course accuracy, different alignment, different identifier conventions, and one critical difference in vertical guidance: an LDA can have a glideslope; an SDF never does.
This post sorts them out the way AIM 1-1-9 and 1-1-10 define them: what each facility is, the numbers (course width, offset limits, usable sector), which can have a glideslope and what that does to the minimums, how each feels on the needle, how to brief the alignment honestly, autopilot considerations, a side-by-side table, and the checkride questions.
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The One-Sentence Difference
Before the details, the frame that keeps them straight.
LDA — a localizer that isn’t aligned with the runway: localizer-comparable accuracy, offset on purpose, possibly with a glideslope
SDF — a simplified, wider localizer-style course that is nearly aligned with the runway: 6° or 12° course width, convergence generally within 3°, never a glideslope
Two axes: accuracy and alignment. The LDA is accurate but offset; the SDF is wide but aligned
If you remember nothing else: LDA = offset, SDF = wide
LDA — Localizer-Type Directional Aid (AIM 1-1-9)
What it is:
A facility of comparable use and accuracy to a localizer, but not part of a complete ILS
The LDA course is NOT aligned with the runway
Course accuracy comparable to a localizer — the AIM contrasts it directly with the SDF’s wider 6° or 12° course; expect localizer-like sensitivity on final
Identified like a localizer: “I” plus three letters (e.g., I-ABC), in Morse on the LDA frequency in the localizer band (108.10–111.95 MHz)
Why it exists:
Installed where a runway-aligned localizer isn’t practical: terrain or obstacles on the extended centerline, restricted or sensitive airspace, noise-abatement corridors, or siting constraints
The designers point the course where it needs to go, and accept that the pilot will align with the runway visually at the end
A few well-known LDAs follow a river corridor into a major airport to keep traffic clear of restricted airspace
Geometry drives the offset; the offset is the point
The 30° rule — the key LDA fact:
Straight-in minimums may be published where the angle between the LDA course and the runway does not exceed 30°
Where the alignment exceeds 30°, only circling minimums are published
So an LDA plate with straight-in minimums tells you the offset is 30° or less; a circling-only LDA tells you it’s more
Read the final approach course against the runway heading and know the number before you fly it
Glideslope — sometimes:
A very limited number of LDA approaches incorporate a glideslope; they’re annotated in the plan view with a note such as “LDA/Glideslope”
An LDA with a glideslope is an approach with vertical guidance (APV), flown to a DA
Without a glideslope, an LDA is a nonprecision approach flown to an MDA
The plate tells you which — brief it
How it feels:
On the needle, an LDA is a localizer: sensitive on final, tightening toward the facility, small corrections, no chasing
The difference from an ILS localizer is entirely in where the course points, not how it behaves
With a glideslope, it’s an ILS-style profile to a DA pointed somewhere other than the runway centerline
Fly the needle like a localizer; manage the alignment like an offset approach
SDF — Simplified Directional Facility (AIM 1-1-10)
What it is:
A facility that provides a final approach course similar to an ILS localizer, with no glide slope information
The course may be wider than a localizer, resulting in less precision: course width is fixed at either 6° or 12°, whichever gives the best flyability at the site
Usable off-course indications are limited to 35° either side of the course centerline; indications beyond that should be disregarded
Identified by a three-letter identifier (no “I” prefix) in Morse, in the same localizer frequency band
Alignment — nearly aligned:
The SDF antenna may be offset from the runway centerline, so the course may converge with the runway at a small angle
That angle is generally not more than 3°
Because the course originates at the antenna, which may sit beside the runway, an approach continued past the threshold leads toward the antenna — not down the centerline
A technicality on breakout, not a visual-maneuvering problem
Why it exists:
A lower-cost, simpler installation than a localizer, giving a runway-oriented course at airports where a full localizer wasn’t justified
A non-RNAV option from the pre-GPS era
Only a handful remain in the U.S.; most have been replaced by RNAV (GPS)
You’ll be asked about it more often than you’ll fly it
Glideslope — never:
An SDF has no glideslope and no approved vertical guidance, ever
It’s always a nonprecision approach to an MDA
Any “+V” your GPS displays on an SDF is advisory only
There is no chart that “explicitly provides approved vertical guidance” on an SDF — don’t look for one
How it feels:
Localizer technique with a lazier needle: the wider course means the same deflection represents a larger lateral error, and the needle responds a bit more slowly than an ILS localizer
More sensitive than a VOR (±10° full-scale), less sensitive than a localizer
Stay centered, keep corrections small, and don’t be lulled by the easy needle
Get inside the 35° sector before trusting the indication
Side by Side
LDA | SDF | |
Full name | Localizer-Type Directional Aid | Simplified Directional Facility |
Course accuracy / width | Localizer-comparable (the AIM contrasts it with the SDF’s 6° or 12°) | Reduced — fixed at 6° or 12° |
Alignment with runway | Offset — straight-in minimums if ≤30°, circling only if >30° | Nearly aligned — generally ≤3° |
Glideslope | Sometimes (LDA/Glideslope — an APV, flown to a DA) | Never (always an MDA) |
Usable indications | Localizer standard | 35° either side of course |
Identifier | I-xxx (like a localizer) | xxx (three letters, no “I”) |
Frequency band | 108.10–111.95 MHz | 108.10–111.95 MHz |
Why it exists | Geometry/airspace constraints prevent an aligned course | Cheaper, simpler course guidance |
The defining feature | Precise but offset | Wide but aligned |
Breakout | Visual alignment maneuver proportional to the offset — can be large | Runway essentially straight ahead |
The two memory hooks:
LDA: the “D” is for Directional — it points somewhere other than the runway
SDF: the “S” is for Simplified — a simpler, wider course that’s nearly aligned
Offset vs. width
Keep them separate
Vertical Guidance: The Biggest Operational Difference
Brief this first.
LDA without glideslope:
Nonprecision: step-down fixes, MDA, MAP, VDP if published
Dive-and-drive or CDFA — both acceptable, step-downs respected, MDA a hard floor
“+V” from a GPS is advisory
Standard nonprecision discipline, plus the offset
LDA with glideslope (LDA/GS):
An APV — approved vertical guidance to a DA
Fly it like an ILS: intercept the glideslope from below, cross-check the altitude at the FAF/glideslope intercept, pitch for the path, decide at DA
At DA the runway may be off to one side; you’ve decided to continue, and now you align — with the same stabilized-approach criteria as any other approach
For alternate planning, an APV uses the nonprecision 800-2 standard (TPP legend), the same as other LDAs and SDFs
SDF:
Always nonprecision, always an MDA
Step-downs, MDA, MAP (often by timing or DME), VDP if published
“+V” advisory only
The one certainty in this comparison: no SDF has vertical guidance
The briefing question:
“Approved vertical guidance — yes or no? DA or MDA?”
LDA: check the plate; SDF: no
Set the altitude selector and the minimums accordingly
Then brief the alignment
Alignment: Brief the Offset Honestly
The difference between a 3° convergence and a 25° offset is the difference between a normal landing and a visual maneuver.
LDA — expect a real maneuver:
Compare the final approach course to the runway heading; the difference is your offset, and it can be anywhere from a few degrees to 30° (or more, circling)
On breakout, the runway will be visibly off to one side; the larger the offset, the more lateral displacement you’ll have at a given distance from the threshold
Plan where you’ll see the runway (the VDP or the DA), how much turn it takes to align, and whether you can be stabilized by your gate — commonly 1,000 feet AGL in IMC, 500 in VMC, and aligned on breakout
If you can’t be aligned and stable in time, go missed — offset approaches punish late, aggressive corrections
LDA circling-only — it’s a circle:
If the plate publishes only circling minimums, the offset exceeds 30° and the procedure is a circling approach: maneuver to the runway within the circling area at or above the circling MDA, with the runway in sight the whole time
Standard circling discipline: don’t descend below the circling MDA until in a position for a normal descent to the runway
Know the circling radius for your category
Not a straight-in
SDF — nearly straight ahead:
With a convergence of 3° or less, the runway appears essentially straight ahead on breakout, slightly off to one side at most
A small heading change aligns you
Brief the convergence and the antenna side (the course leads to the antenna past the threshold), but don’t over-dramatize it
The discipline is stabilization and the MDA, not visual maneuvering
For both — the go-around trigger:
If the runway isn’t where you expected, if alignment requires more than a normal maneuver, or if you aren’t stabilized by your gate: missed approach
“Salvaging” a landing with a low, steep turn is the setup for a loss-of-control or a hard landing
The missed approach course is built around the final approach course, so fly it as published
Decide the trigger before the FAF
Autopilot and Automation
Lateral:
Both LDA and SDF are localizer-band signals; most autopilots will couple laterally in NAV/LOC mode
An LDA tracks like a localizer; an SDF may hunt a little on the wider course
Set the final approach course on the HSI/CDI for reference (the OBS doesn’t affect a localizer-type signal, but the course pointer orients the HSI)
Monitor the tracking either way
Vertical:
LDA/GS: the autopilot’s approach mode can couple the glideslope like an ILS, if the installation is approved for it — check the AFM supplement
LDA without GS and all SDFs: no vertical mode to couple; descent is yours — vertical speed or pitch mode, altitude preselect at each step-down and then the MDA
Never couple to an advisory “+V” as if it were a glideslope
Know which you have before the FAF
Database loading:
Load the approach from the GPS database by name for fix identification and situational awareness, but navigate on the ground-based signal — these aren’t RNAV approaches
The database will show the offset course on the map, which makes the alignment picture obvious before you fly it
Confirm the fixes, step-downs, and MAP against the plate
The map is a planning aid; the needle is the navigation
The automation trap:
Automation can fly a wide SDF course or an offset LDA course accurately and leave you unaware of what the course actually is
Always know: what lateral mode you’re in, whether you have approved vertical guidance or just advisory cues, and where the course points relative to the runway
Brief it out loud
The autopilot doesn’t know the runway is 25° to the left
A Briefing Template
When you see LDA or SDF on the plate, brief in this order.
1. Facility and alignment:
“LDA — localizer-accurate, offset [X]° from the runway, straight-in (or circling) minimums”
or “SDF — wider course, 6° or 12°, converges [X]° with the runway, nearly aligned”
Frequency, identifier (I-xxx or xxx), usable sector (35° for SDF)
Where the course points relative to the runway and which side the antenna is on
2. Vertical plan:
“Approved vertical guidance? LDA/GS → DA ___. Otherwise → MDA ___”
Step-down fixes and altitudes, FAF, how the MAP is defined (fix / DME / time), VDP
“+V” is advisory
Altitude preselects
3. Visual transition:
“Expect the runway [straight ahead / off to the left by about X°] at [the VDP / DA]”
The alignment maneuver and the stabilized-approach gate
Go-around trigger: “not aligned and stable by ___ → missed”
For circling-only LDA: the circling plan
4. Missed approach:
Read it twice; offset courses often have missed approaches that turn away from the final approach course
Initial heading, altitude, holding fix
Climb first, then navigate
Especially important on offset courses
Worked Scenarios
Scenario 1 — LDA with a 20° offset, no glideslope:
LDA RWY 27; final approach course 250°, runway 270°; straight-in minimums (offset ≤30°); MDA 1,100; MAP by DME; VDP published
Brief: localizer-accurate needle, 20° offset, MDA approach, expect the runway about 20° to the right at the VDP, align with a shallow right turn, go missed if not stable by the gate
Fly it as a localizer to the MDA; at the VDP the runway appears right of the nose; a gentle right turn aligns; stabilized; land
If the runway appears later than the VDP, the alignment turn gets tighter and lower — a missed approach is likely the right call
Scenario 2 — LDA/GS:
Same geometry, but the plate carries a glideslope and a DA of 850
Brief: APV, DA approach, ILS technique to the DA, then a 20° visual alignment from 850 feet
Intercept the glideslope from below, cross-check the FAF altitude, decide at DA
At DA with the runway in sight to the right: continue and align; without it: missed — the DA is the decision, the alignment is the next five seconds
Scenario 3 — SDF:
SDF RWY 18; course 183°, runway 180° (3° convergence); course width 12°; MDA 1,020; MAP at the threshold by DME; no glideslope; GPS shows “+V”
Brief: wide course, nearly aligned, MDA approach, +V advisory, runway essentially straight ahead on breakout
Localizer technique with small corrections; constant descent that respects the step-down; level at the MDA; runway appears slightly off to one side at 1.5 NM; small heading change; land
No visual maneuver to speak of — the discipline is the MDA
Scenario 4 — the misidentification:
A pilot briefs an LDA with a 28° offset as “localizer-like, basically straight-in” because it has straight-in minimums
On breakout at the MDA the runway is well off to the left; the pilot attempts a low, steep turn to align
The straight-in minimums meant the offset was ≤30°, not that it was small — 28° is a real maneuver that needed to be planned
Brief the number, not the category
Common Pilot Mistakes
Treating LDA and SDF as the same thing (“the offset ones”) — the LDA is offset, the SDF is wide
Assuming an SDF might have approved vertical guidance (it never does)
Not checking whether an LDA has a glideslope (and therefore whether it’s a DA or an MDA)
Reading “straight-in minimums” on an LDA as “small offset” — it means ≤30°, which can still be a significant maneuver
Flying a circling-only LDA as a straight-in
Chasing the SDF’s wide needle, or over-correcting on it like an ILS
Trusting SDF indications from outside the 35° usable sector
Following the SDF course past the threshold toward the antenna
Descending on a “+V” as if it were a glideslope
“Salvaging” an offset breakout with a low, steep turn instead of going missed
Not briefing the missed approach on an offset course
Checkride Questions — With Answers
“What’s the difference between an LDA and an SDF?”
An LDA has localizer-comparable accuracy but is not aligned with the runway — straight-in minimums are published when the offset is 30° or less, circling only beyond that — and a few LDAs have a glideslope. An SDF is a simplified, wider course (fixed at 6° or 12°) that is nearly aligned with the runway (generally within 3°), with usable indications limited to 35° either side, and never has a glideslope.
“Which one can have vertical guidance?”
Only the LDA. An LDA with a glideslope (annotated “LDA/Glideslope”) is an APV flown to a DA. An SDF is always a nonprecision approach to an MDA.
“What does it mean if an LDA has straight-in minimums?”
The angle between the LDA course and the runway is 30° or less. More than 30° → circling minimums only.
“How wide is an SDF course?”
Fixed at 6° or 12°, versus a localizer’s 3°–6° tailored to about 700 feet at the threshold.
“How are they identified?”
LDA: “I” plus three letters, like a localizer. SDF: a three-letter identifier with no “I.”
“What’s the SDF’s usable sector?”
35° either side of the course centerline; disregard indications beyond that.
“How do you brief the alignment?”
Compare the final approach course to the runway heading. LDA: expect a visual alignment maneuver proportional to the offset (or a circle if >30°), with a stabilized-approach gate and a go-around trigger. SDF: expect the runway nearly straight ahead; a small heading change.
Final Thoughts
LDA and SDF approaches both put a localizer-style needle in front of you, which is why pilots merge them in memory. Pulling them apart takes one idea: the LDA is a real localizer pointed somewhere other than the runway, and the SDF is a simpler, wider localizer pointed almost exactly at it. Everything else follows — the LDA’s 30° straight-in rule and its visual alignment maneuver, the SDF’s 6° or 12° width and its 35° usable sector, and the fact that an LDA can carry a glideslope (making it an APV to a DA) while an SDF never does.
Brief the facility type and the actual offset number, decide whether you’re flying to a DA or an MDA, plan the visual transition and the go-around trigger, read the missed approach twice, and fly smooth corrections on whichever needle you’ve got.
On the Written Test and Checkride
LDAs and SDFs appear on the instrument knowledge test and in instrument orals, usually together. The most commonly tested topics:
LDA: localizer-comparable accuracy, not aligned, ≤30° straight-in / >30° circling, sometimes a glideslope (APV)
SDF: 6° or 12° width, ≤3° convergence, 35° usable sector, never a glideslope
Identifiers (I-xxx vs. xxx) and the shared localizer frequency band
DA vs. MDA by facility
Offset briefing and the stabilized-approach go-around decision
Quick Reference
The Frame:
LDA = precise but offset. SDF = wide but aligned.
Only the LDA can have a glideslope
LDA (AIM 1-1-9):
Localizer-comparable accuracy (the AIM contrasts it with the SDF’s wider 6° or 12° course); not part of a complete ILS
Not aligned with the runway — straight-in minimums if offset ≤ 30°; circling only if > 30°
Sometimes a glideslope (“LDA/Glideslope” note) → APV, DA; otherwise NPA, MDA
Identifier I-xxx; 108.10–111.95 MHz
Breakout: visual alignment maneuver proportional to the offset; brief the number
SDF (AIM 1-1-10):
Course width fixed at 6° or 12° → less precision than a localizer, more than a VOR
Nearly aligned — convergence generally ≤ 3°; course originates at the antenna (leads to the antenna past the threshold)
Usable indications 35° either side; disregard beyond
Never a glideslope → always MDA; “+V” advisory only
Identifier xxx (no “I”); 108.10–111.95 MHz
Breakout: runway essentially straight ahead
Vertical Plan:
LDA/GS: ILS technique to DA, then align
LDA (no GS) and SDF: step-downs, MDA, MAP, VDP; dive-and-drive or CDFA; “+V” advisory
Alternate planning: all of these use the nonprecision/APV 800-2 standard
Autopilot:
Lateral: NAV/LOC couple on both; SDF may hunt on the wide course
Vertical: LDA/GS may couple like an ILS if approved; otherwise manual — preselect step-downs, then MDA
Load from the database for the map picture; navigate on the ground signal
Briefing Order:
Facility and alignment (type, offset/convergence number, straight-in vs. circling, identifier, usable sector)
Vertical plan (DA or MDA, step-downs, MAP definition, VDP, preselects)
Visual transition (where the runway will appear, the alignment maneuver, the stabilized gate, the go-around trigger)
Missed approach (read twice — often turns away from the final course)
Key Principle:
The LDA is a localizer pointed away from the runway — localizer-accurate, offset up to 30° with straight-in minimums (circling beyond), occasionally with a glideslope that makes it an APV to a DA. The SDF is a simpler, wider localizer pointed almost at the runway — 6° or 12° wide, usable within 35°, converging 3° or less, and never with a glideslope. Brief the type and the actual offset number, know whether you’re flying to a DA or an MDA, plan the alignment and the go-around trigger, and fly smooth corrections.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.