SDF Approaches Explained: How a Simplified Directional Facility Differs from a Localizer and an LDA, Course Width, Offset, and Flying It to MDA
An SDF — Simplified Directional Facility — is one of those instrument approach terms that feels like a relic until you’re flying into a smaller airport in marginal weather and the only non-RNAV option on the plate says SDF RWY XX. Only a handful of them survive in the United States, and RNAV (GPS) has replaced most of the rest. But the SDF is still on the instrument knowledge test and in instrument orals, because it sits at the center of a question every instrument pilot should be able to answer: what exactly is the difference between a localizer, an LDA, and an SDF?
The short version: all three give you a localizer-style needle, and all three are flown to an MDA when there’s no glideslope. The differences are in course width, alignment with the runway, and whether vertical guidance can ever exist. An SDF is the widest, is nearly aligned with the runway (generally within 3 degrees), and never has a glideslope. This post covers what an SDF is and how it’s built, the numbers from AIM 1-1-10, how it compares to a localizer and an LDA, the alignment issue that actually matters, the no-glideslope discipline (including what to do with an advisory “+V”), cockpit setup, and the checkride questions.
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What an SDF Is
The AIM 1-1-10 definition, unpacked.
The basics:
A ground-based facility that provides a final approach course similar to an ILS localizer
Lateral guidance only — it does not provide glide slope information
The approach techniques are essentially the same as a standard localizer approach, with two exceptions: the course may not be aligned with the runway, and the course may be wider, resulting in less precision
“Simplified” means simpler and cheaper to install than a localizer, at the cost of precision
Frequency and identification:
Transmits in the localizer band: 108.10 to 111.95 MHz
Identified by a three-letter identifier in Morse code on the SDF frequency (a localizer’s identifier begins with “I” — an SDF’s does not)
Tuned and identified exactly like a localizer on any nav receiver that handles LOC frequencies
The approach chart shows the identifier and frequency
What it’s for:
A runway-oriented, localizer-style course at airports where a full ILS or localizer wasn’t justified or couldn’t be sited
A non-RNAV option alongside (or before) GPS approaches
Lower cost and simpler siting than a localizer
A legacy of the pre-GPS approach network
Where it stands today:
Only a few SDF approaches remain in the U.S.; the rest have been decommissioned in favor of RNAV (GPS)
You may never fly one — but you’ll be asked about one
Treat it as a localizer approach with a wider course and no glideslope, and you’ll fly it correctly
The knowledge matters more than the frequency of use
The Numbers That Define It (AIM 1-1-10)
What “less precision” means, in specifics.
Course width — fixed at 6° or 12°:
An SDF’s course width is set at either 6 degrees or 12 degrees, whichever gives the best flyability and course quality at that site
A localizer’s course width is tailored between 3° and 6° so that full-scale deflection is about 700 feet wide at the runway threshold
So an SDF is as wide as the widest localizer, or twice that
The same needle deflection represents a larger lateral error on an SDF than on a localizer
Usable indications — 35° either side:
Usable off-course indications are limited to 35 degrees either side of the course centerline
Instrument indications received beyond 35 degrees should be disregarded
A full-scale needle well off the course does not mean “turn hard toward the needle” — it means you’re outside the usable area
Intercept the course from inside the 35° sector before trusting the indication
Alignment — generally within 3°:
The SDF antenna may be offset from the runway centerline, so the final approach course may converge with the runway at a small angle
That angle is generally not more than 3 degrees — determine it from the approach chart by comparing the final approach course to the runway heading
A 3° difference is barely perceptible on breakout
The LDA is the procedure with large offsets (below)
The antenna-site catch:
Because the approach course originates at the antenna, which may sit beside the runway rather than on its extended centerline, an approach continued beyond the threshold leads the aircraft toward the antenna’s offset position — not along the centerline
The course is reliable to the runway; past the threshold, it isn’t the runway
Transition to visual references before the threshold, as on any approach
Don’t track the needle into the flare
Comparison to a VOR:
A VOR approach’s full-scale deflection is 10° either side of course (20° total)
An SDF at 6° or 12° total is tighter than that
So the SDF needle is more sensitive than a VOR needle, less sensitive than a localizer needle
Fly it with localizer-style small corrections, expecting a slightly slower needle response than an ILS
SDF vs. Localizer vs. LDA
The comparison examiners ask for.
Localizer (LOC) | LDA | SDF | |
What it is | The lateral component of an ILS (usable alone as a LOC approach) | Localizer-type Directional Aid — a localizer that’s not part of a complete ILS and not aligned with the runway | Simplified Directional Facility — a simpler, wider localizer-style course |
Course width | 3°–6°, tailored to ~700 ft at the threshold | 3°–6° (localizer accuracy) | Fixed at 6° or 12° |
Alignment with runway | Aligned | Offset — straight-in minimums may be published if the offset is 30° or less; circling only if more than 30° | Nearly aligned — generally within 3° |
Glideslope | Yes on an ILS; a LOC-only approach has none | Some LDAs have a glideslope (LDA/GS, an APV flown to a DA) | Never |
Minimums | DA (ILS) or MDA (LOC) | MDA, or DA where a glideslope is published | MDA only |
Frequency band | 108.10–111.95 MHz | 108.10–111.95 MHz | 108.10–111.95 MHz |
Identifier | “I” + three letters | “I” + three letters | Three letters (no “I”) |
Usable indications | Localizer standard | Localizer standard | 35° either side of course |
The one-line distinctions:
A LOC approach is an ILS without the glideslope — aligned, precise, flown to an MDA
An LDA is a localizer-accuracy course that is NOT aligned with the runway — possibly offset a lot (up to 30° with straight-in minimums), possibly with its own glideslope
An SDF is a wider, simpler localizer-style course that IS nearly aligned with the runway — and never has a glideslope
Width and alignment are the two axes; the SDF is wide-and-aligned, the LDA is narrow-and-offset
The common confusion:
Pilots lump LDA and SDF together as “the offset ones”
The offset is the LDA’s defining feature; the SDF’s defining feature is reduced precision (width)
An SDF’s 3° convergence is a technicality; an LDA’s 20° offset is a visual-maneuvering problem
Keep them separate
How It Flies in the Cockpit
Technique is localizer technique, with the width in mind.
Setup:
Tune the SDF frequency on the nav radio and identify the three-letter Morse identifier
On a CDI, the needle behaves like a localizer — the OBS setting has no effect on the indication (set the final approach course anyway as a reference)
On an HSI, set the final approach course on the course pointer so the deviation bar is oriented correctly
Load the approach from the GPS database if available for situational awareness, but navigate on the SDF signal — it’s not an RNAV approach
Intercept:
Get inside the 35° usable sector before trusting the needle — on vectors, ATC will put you there; on a procedure turn or course reversal, the published procedure does
Intercept at a shallow angle (30° or less), as for a localizer
Expect the needle to come alive a little later and move a little slower than an ILS localizer at the same distance, because the course is wider
Don’t over-correct on the first movement
Tracking:
Small heading corrections — 2° to 5° — and wait for the needle to respond
No needle chasing inside the FAF
The wider course means a half-scale deflection is a bigger lateral error than on an ILS; stay centered
Configure early so you’re not changing pitch, power, and heading at once on final
Autopilot:
Most autopilots will couple laterally in NAV/LOC mode (it’s a localizer-band signal)
There is no vertical mode to couple to — descent is your job, via vertical speed or pitch, with the altitude preselector set to each step-down and then the MDA
Treat it exactly as you’d treat a LOC-only approach on autopilot
Monitor the lateral tracking; an autopilot can hunt on a wide course
No Glideslope: The Discipline
The most common error mindset is “the lateral guidance looks precise, so I’ll treat it like an ILS.”
It’s a nonprecision approach:
No glideslope, no DA — you descend to and level at the MDA
The step-down fixes and their altitudes are the vertical structure
The MDA is a hard floor: you may not descend below it unless you have the runway environment in sight and meet 91.175(c)
All the standard nonprecision rules apply
Plan the vertical profile:
Identify every step-down fix (DME, cross-radial, GPS-defined, or timed) and its altitude
Identify the FAF and the MDA
Compute (or read from the chart) the VDP if one is published, or estimate where a normal descent from the MDA would reach the runway
Decide: a “dive and drive” to each step-down, or a constant descent (CDFA) that respects every step-down — either is fine; busting a step-down is not
Constant-descent technique (CDFA) to an MDA:
A planned, continuous descent from the FAF that crosses each step-down at or above its altitude and arrives at the MDA near the VDP
Stabilized, and it keeps the energy state constant
The catch: an MDA is still an MDA — you may not go below it on the descent path unless you have the visual references; many operators add a small buffer (e.g., +50 feet) to the MDA as a “derived DA” so a normal level-off doesn’t bust it
If you reach the MDA without the runway in sight, level off and continue to the MAP (don’t go missed early unless your SOP calls for it at the derived DA)
The “+V” advisory glidepath:
Many GPS navigators display an advisory vertical path (“+V”) on nonprecision approaches, including SDF procedures loaded from the database
It is ADVISORY only — it has no obstacle-clearance guarantee, it isn’t a glideslope, and it doesn’t change the MDA into a DA
Use it to fly a smooth descent, but the step-down altitudes and the MDA govern — if +V would take you below a step-down or the MDA, the altitudes win
An SDF never has approved vertical guidance, so “+V” is always advisory on one
Timing and distance:
If the MAP is defined by time from the FAF, start the clock at the FAF and know your groundspeed-based time
If it’s defined by DME or a GPS fix, know the distance
Know the MAP before you cross the FAF
The missed begins at the MAP, not where you decide the weather is bad
Alignment and the Visual Transition
Framed correctly.
What you’ll actually see:
With a convergence angle of 3° or less, the runway will appear essentially straight ahead on breakout — slightly off to one side at most
A small heading change aligns you
Not a dramatic visual maneuver
Brief it, but don’t over-dramatize it
Why the chart matters:
Compare the final approach course to the runway heading on the plate; the difference is your convergence angle
Note which side the antenna is on (the course will lead toward it past the threshold)
Know what “aligned” will look like
Thirty seconds of briefing
The real discipline — stabilized approach:
Whatever the offset, if you’re not stabilized (on speed, configured, on a normal descent path) by your stabilized-approach gate — commonly 1,000 feet AGL in IMC and 500 feet AGL in VMC under most SOPs — and aligned with the runway on breakout at the MDA, go missed
Being “almost lined up” at a few hundred feet in the IMC-to-VMC transition is not a plan
A low-altitude “dive and yank” to a runway that’s off to the side is the setup for a loss-of-control or a hard landing
The missed approach is free; the rushed landing isn’t
The missed:
Know it cold before the FAF
The missed approach course and initial heading are built for the SDF final approach course; fly them as published
Climb first, then navigate, then talk
Standard missed-approach discipline
Worked Scenario
The setup:
SDF RWY 18 at a small airport; course 183°, runway heading 180° (3° convergence); course width 12°; FAF 5 NM out at 2,400; step-down at 2.5 NM at 1,600; MDA 1,020; MAP at the threshold by DME; no glideslope; GPS shows “+V”
Vectors to a 30° intercept outside the FAF, 3,000 feet
Flying it:
Tune, identify (three-letter Morse), set 183° on the HSI, confirm GPS shows the approach for reference. Inside the 35° sector; needle alive; intercept with small corrections. Configured and at approach speed before the FAF
At the FAF: start a planned descent — constant descent that will cross 2.5 NM at or above 1,600 and reach the MDA near the VDP. The +V may suggest a path; the step-down and MDA govern. Altitude preselect to 1,600, then 1,020
At the MDA with the runway not yet in sight: level, continue to the DME MAP. Runway appears at 1.5 NM, slightly off to one side (the 3° convergence); small heading change to align; stabilized; land
If not in sight at the MAP, or not stabilized on breakout: published missed
Common Pilot Mistakes
Treating a good-looking needle as an ILS and descending below the MDA on an advisory glidepath
Trusting needle indications from outside the 35° usable sector and chasing them
Over-correcting on a wide course (treating a half-scale deflection like an ILS half-scale deflection)
Confusing the SDF’s small convergence with an LDA’s large offset — or lumping LDA and SDF together as “the same thing”
Busting a step-down fix on a “constant descent”
Not knowing how the MAP is defined (time vs. DME vs. GPS fix)
Tracking the course past the threshold toward the antenna instead of the centerline
Rushing a late alignment near minimums instead of going missed
Expecting a glideslope that will never appear
Checkride Questions — With Answers
“What is an SDF?”
A Simplified Directional Facility: a ground-based facility that provides a final approach course similar to a localizer, with no glideslope. The course may be wider (fixed at 6° or 12°) and may not be exactly aligned with the runway (generally within 3°). Flown like a localizer approach to an MDA.
“How does an SDF differ from a localizer?”
Wider course — 6° or 12°, versus a localizer’s 3°–6° tailored to about 700 feet at the threshold — so less precision; possibly a small misalignment with the runway; usable indications limited to 35° either side; no “I” prefix on the identifier; never a glideslope.
“How does an SDF differ from an LDA?”
An LDA has localizer accuracy (3°–6° width) but is NOT aligned with the runway — offset up to 30° with straight-in minimums, more than that with circling only — and some LDAs have a glideslope. An SDF is wider, nearly aligned, and never has a glideslope.
“Is an SDF a precision approach?”
No. It’s a nonprecision approach flown to an MDA. Any “+V” displayed by a GPS is advisory only.
“What’s the usable range of the SDF course?”
35° either side of the course centerline; indications beyond that should be disregarded.
“What frequency band does an SDF use, and how is it identified?”
108.10–111.95 MHz (the localizer band); a three-letter Morse identifier on the SDF frequency.
“What happens if you follow the SDF course past the runway threshold?”
Because the course originates at the antenna, which may be offset from the centerline, continuing past the threshold leads toward the antenna site, not down the runway.
Final Thoughts
The SDF is a localizer’s simpler cousin: the same needle, the same technique, a wider course, a near-aligned final, and never a glideslope. You’ll rarely fly one, but understanding it sharpens the whole family — localizer, LDA, SDF — into a clean picture of width, alignment, and vertical guidance, which is exactly what the knowledge test and the oral are probing.
In the airplane, the two priorities are simple: don’t let a precise-looking needle talk you into ILS behavior — the step-downs and the MDA govern, and “+V” is advisory — and fly a stabilized approach with the convergence angle briefed, willing to go missed the moment alignment or stabilization isn’t there. Do that and an SDF is just another localizer approach with a slightly lazier needle.
On the Written Test and Checkride
SDFs appear on the instrument knowledge test and in instrument orals, usually alongside the LDA. The most commonly tested topics:
Definition: localizer-like course, no glideslope, possibly wider and possibly misaligned
Course width fixed at 6° or 12° vs. a localizer’s 3°–6°
Usable indications limited to 35° either side
Convergence with the runway generally ≤3°; the antenna-offset catch past the threshold
SDF vs. LDA (width vs. offset; LDA may have a glideslope, SDF never)
Nonprecision — flown to an MDA; “+V” is advisory
Frequency band and three-letter identifier
Quick Reference
SDF — Simplified Directional Facility (AIM 1-1-10):
Localizer-like final approach course; no glideslope — ever
Flown like a localizer approach to an MDA
Frequency: 108.10–111.95 MHz; identifier: three-letter Morse (no “I”)
Course width fixed at 6° or 12° (localizer: 3°–6°, ~700 ft at threshold) → less precision
Usable off-course indications: 35° either side; disregard beyond that
Alignment: antenna may be offset; convergence with the runway generally ≤3° — check the chart
The course originates at the antenna → past the threshold it leads to the antenna site, not the centerline
LOC vs. LDA vs. SDF:
LOC | LDA | SDF | |
Width | 3°–6° | 3°–6° | 6° or 12° |
Alignment | Aligned | Offset (≤30° straight-in; >30° circling) | ≈Aligned (≤3°) |
Glideslope | ILS yes / LOC-only no | Sometimes (LDA/GS) | Never |
Minimums | DA / MDA | MDA (DA with GS) | MDA |
Identifier | I-xxx | I-xxx | xxx |
LDA = precise but offset; SDF = wide but aligned
Cockpit Setup:
Tune, identify; set the final approach course on HSI/CDI (OBS has no effect on a LOC-type signal)
Load the GPS procedure for awareness; navigate on the SDF
Intercept inside the 35° sector, shallow angle; expect a slower needle than an ILS
Autopilot: lateral couple OK (LOC mode); vertical is manual — preselect each step-down, then the MDA
Vertical Discipline:
Step-downs are hard altitudes; MDA is a hard floor (91.175(c) to go below)
Dive-and-drive or CDFA — both fine; busting a step-down is not
CDFA to an MDA: consider a derived-DA buffer; level at the MDA and continue to the MAP if not visual
“+V” is advisory only — altitudes govern
Know how the MAP is defined (time / DME / GPS fix) before the FAF
Alignment & Stabilization:
Compare final approach course to runway heading; note the antenna side
≤3° convergence → runway nearly straight ahead; small heading change to align
Not stabilized by your gate (commonly 1,000 ft AGL IMC / 500 ft AGL VMC) or not aligned on breakout → go missed; no dive-and-yank
Know the missed cold
Key Principle:
An SDF is a localizer approach with a wider course (6° or 12°), usable only within 35° of centerline, nearly aligned with the runway (≤3°), and never a glideslope. Fly it with localizer technique to an MDA: respect the step-downs, keep “+V” advisory, brief the small convergence, and go missed if the visual transition isn’t stable.
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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.