terps for pilots - pbpt · for approaches, osc slope = 102 / glideslope angle or glideslope angle =...

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!

• That means I probably cannot answer specific questions about 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:

Θ descent = tan-1 { r + alt r + THRe + TCH } . r

D fix } . { 180

π ln

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

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

4000 feet

1000 feet

IFR Obstacle Clearance – Enroute and initial approach segments

IFR ROC 1000 feet 500 feet

Primary OC area Secondary OC area

8 Miles 2 Miles

Federal Airway or Initial Approach

Primary OCA – 8 miles

Secondary OCA – 2 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

Secondary OC area

Primary 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

OCS

ROC

Segments of an Instrument Approach #1

IF FAF MAP MAF

Level OCS with Constant ROC - VOR Approach

1,000 ft 500 ft 250 ft

250 ft

IF

FAF Stepdown

MAP

IAF

Segments of an Instrument Approach #2

IF FAF DH MAF

LOC

Sloping OCS

OCS

DH (HAT)

(Used for precision approaches, LPV approaches, and departure procedures). NOT USED FOR LNAV/VNAV

approaches! VNAV does not provide obstacle clearance!

ROC

For approaches, OSC slope = 102 / glideslope angle or glideslope angle = 102 / OCS slope

Those Darned Trees!

OCS

DH (HAT)

ROC

Four solutions for the problem of obstacle penetration – from worst to best….

Solution #1 – Increase the DH (The worst solution – and only allowed for existing

obstacles)

OCS

Solution #2 – Increase the OCS angle

OCS

This requires increasing the Glidepath angle – possibly reducing the airport utility.

Solution #3 – Displace the Runway Threshold

OCS

Reduces the effective length of the runway for landing – reducing the airport utility

New Threshold

Solution #4 – Best - Remove the Obstacle

OCS

DH (HAT)

The most permanent solution

The Bigger Problem (most of the time)

OCS DH

(Precision Approach shown)

ROC

OUCH!

What’s the solution?

Airport

Solution #1 – Increase DH or MDA

OCS

DH ROC

This solution increases the ceiling and visibility minimums for all aircraft on the

approach

Airport

Solution #2 – Increase OCS Angle

OCS

DH ROC

Missed approach requires a minimum climb of XXX feet per NM to XXXX. If unable to meet climb gradient (options)

Airport

(This solution may limit the type of aircraft able to use the approach).

Why are the Non-Precision Approach minimums higher than expected?

OCS

DH ROC

Airport

It often has to do with the location of the MAP

Why are the Non-Precision Approach minimums higher than expected?

OCS

MAP

Airport

You get much closer to the airport (and the obstructions in the missed approach area) on the non-precision approach

FAF

Remember! VNAV indications and VDP points don’t guarantee obstruction clearance (but VASI and PAPI do)

Important Takeaways

• Follow established procedures!

• If you can’t maintain the required minimum climb gradient, you are not assured of adequate obstacle clearance!

• Min 200 feet per mile for Departures and Missed approaches – can sometimes be more

• Enroute performance minimums: • SL to 5000 feet – 150 ft./NM • 5000 to 10,000 feet – 120 ft./NM • Above 10,000 feet – 100 ft./NM

• Think long and hard about the wisdom of IFR flight if you can’t climb at these rates…..

Thanks for Listening

• Questions ?