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```TERPS for PILOTS
John Andrick
BPT, Inc.
INTRODUCTION
• This course will introduce the pilot to elementary principles of
obstacle analysis for the most popular instrument procedures today.
• We’re not going to get into the “nuts and bolts” of creating
instrument procedures – even the feds disagree on some of this stuff!
specific approaches.
• The goal is to make you aware of the various metrics used in obstacle
clearance analysis so you will understand the risk you’re taking if you
depart from standard procedures.
Reason we’re not going too deep:
{ ln {
Θ descent = tan-1
r + alt
r + THRe + TCH
}.
r
D fix
}.
Where:
• Alt = FAF altitude
• THRe = Threshold elevation in feet above sea level
• r = radius of the earth at the FAF in feet
• TCH = Threshold Crossing Height
• D fix = Distance from FAF to MAP in feet
180
π
DEFINITIONS
• Precision Approach - Approach with electronic vertical guidance that
meets the requirements of ICAO Annex 10.
• ILS and PAR (DH published)
• (Why not LPV? ICAO needs to move into the present!)
• Approach with Vertical Guidance - Approach with electronic vertical
guidance that does not meet the requirements of ICAO Annex 10.
• LPV, LNAV/VNAV, and LDA with Glideslope (DA published)
• Non-Precision Approach
• Everything else (MDA published)
Definitions – cont.
• OCS – Obstacle Clearance Surface
• An imaginary plane determined by the highest obstacle within a defined
lateral or longitudinal space. No obstacles are permitted above the OCS.
• Can be level or sloping, depending on phase of flight.
• ROC – Required Obstacle Clearance
• The minimum vertical separation needed between the OCS and the airplane.
• Can be constant, increasing or decreasing, depending on phase of flight.
• These concepts are familiar to the VFR pilot, but they are applied in a
different way for IFR operations.
• FAR 91.119
VFR Obstacle Clearance
1000 feet
4000 feet
IFR Obstacle Clearance – Enroute and initial
approach segments
Primary OC area
Secondary OC area
500 feet
2 Miles
IFR ROC 1000 feet
8 Miles
Federal Airway or Initial Approach
Secondary OCA – 2 miles
Primary OCA – 8 miles
IFR Obstacle Clearance – Intermediate, Final,
and Missed Approach segments
IFR ROC
Secondary OC area
(width varies)
Primary OC area
(width varies)
General View – Final Approach Segment
Primary OC area
Secondary OC area
Standard Dimensions
The dimensions of the final approach trapezoid are
defined by the type of facility used for lateral
guidance in the approach:
VOR – 30 miles long. Primary area is 2 miles wide at
the facility and expands to 5 miles wide at the 30
mile point. Secondary widths are 0 to 1 mile.
NDB – 15 miles long. Primary area is 2.5 miles wide
at the facility and expands to 5 miles wide at the 15
mile point. Secondary widths are 0 to 1.34 miles.
LOC/ILS/LP/LPV – 50,000 feet long. 1,400 feet wide
at the narrow end (200 feet from the runway
threshold) increasing to 2 nm wide at the 50,000
foot point. Secondary areas are 300 feet wide
increasing to 2,500 feet.
LNAV or LNAV/VNAV – No length specified.
Primary width is 4 NM at the FAF decreasing to 1.2
NM at the MAP. Secondary areas are 1 NM wide.
Standard ROC’s
• Enroute – 1,000 / 2,000 feet (possibly 1500/1700 feet with waiver)
• Initial Approach – 1000 feet
• Intermediate Approach – 500 feet
• Final Approach
•
•
•
•
NDB without FAF – 350 feet
VOR without FAF or NDB with FAF – 300 feet
LOC, LDA, LNAV, LNAV/VNAV, LP, VOR with FAF – 250 feet
ILS and LPV – 200 feet HAT (we will not consider cat 2/3 in this course)
Level OCS with Constant ROC - Enroute
ROC
OCS
Segments of an Instrument Approach #1
IF
FAF
MAP
MAF
Level OCS with Constant ROC - VOR Approach
IAF
IF
FAF
1,000 ft
Stepdown
MAP
500 ft
250 ft
250 ft
Segments of an Instrument Approach #2
LOC
IF
FAF
DH
MAF
Sloping OCS
(Used for precision approaches, LPV approaches, and
departure procedures). NOT USED FOR LNAV/VNAV
approaches! VNAV does not provide obstacle clearance!
For approaches, OSC slope = 102 / glideslope
angle or glideslope angle = 102 / OCS slope
ROC
DH (HAT)
OCS
Those Darned Trees!
Four solutions for the problem of obstacle
penetration – from worst to best….
ROC
DH (HAT)
OCS
Solution #1 – Increase the DH
(The worst solution – and only allowed for existing
obstacles)
OCS
Solution #2 – Increase the OCS angle
This requires increasing the
Glidepath angle – possibly
reducing the airport utility.
OCS
Solution #3 – Displace the Runway Threshold
Reduces the effective length
of the runway for landing –
reducing the airport utility
New Threshold
OCS
Solution #4 – Best - Remove the Obstacle
The most permanent solution
DH (HAT)
OCS
The Bigger Problem (most of the time)
(Precision Approach shown)
What’s the solution?
OUCH!
ROC
DH
OCS
Airport
Solution #1 – Increase DH or MDA
This solution increases the ceiling and
visibility minimums for all aircraft on the
approach
ROC
DH
OCS
Airport
Solution #2 – Increase OCS Angle
Missed approach requires a minimum climb of XXX feet per NM
to XXXX. If unable to meet climb gradient (options)
(This solution may limit the type of aircraft able to use the approach).
ROC
DH
OCS
Airport
Why are the Non-Precision Approach
minimums higher than expected?
It often has to do with the location of the MAP
ROC
DH
OCS
Airport
Why are the Non-Precision Approach
minimums higher than expected?
You get much closer to the airport (and the obstructions in the missed
approach area) on the non-precision approach
Remember! VNAV indications and VDP points don’t guarantee obstruction
clearance (but VASI and PAPI do)
FAF
MAP
OCS
Airport
Important Takeaways
• If you can’t maintain the required minimum climb gradient, you are not