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Advisory Circular (AC) No. 500-020 Issue 1

Flight Management System (FMS) Barometric Vertical Navigation (VNAV) Temperature Compensation

File No.: 5009-32-4 AC No.: 500-020
RDIMS No.: 1441506-V7 Issue No.: 01
Issuing Branch: Aircraft Certification Effective Date: 2006-02-10

1.0 Introduction

1.1 Purpose
1.2 Guidance Applicability
1.3 Description of Changes
1.4 Termination

2.0 References

2.1 Reference Documents
2.2 Cancelled Documents
2.3 Definitions

3.0 Background

4.0 New or Updated fms designs

4.1 New/Updated FMS Designs Incorporating Barometric VNAV Approach Capability
4.2 Enabling Temperature Compensation
4.3 Prevention of Incorrect Temperature Selection
4.4 Manually Entered Waypoint Altitudes
4.5 Annunciation and Indication
4.6 Means of Temperature Correction for Barometric Altitude
4.7 Simplified Method
4.8 Accurate Method

5.0 Installation of Existing FMS without temperature compensation

6.0 Headquarters Contact

1.0 Introduction

This Advisory Circular (AC) describes an acceptable means, but not the only means of demonstrating compliance with regulations and standards. This AC in and of itself does not change, create, amend or permit deviations from regulatory requirements nor does it establish minimum standards.  The applicant may elect to follow an alternate method, which must be acceptable to Transport Canada.

1.1 Purpose

The purpose of this Advisory Circular is to provide Headquarters and Regional Aircraft Certification personnel, delegates and industry with guidance on the criteria for incorporation of temperature compensation in new or updated Flight Management System (FMS) designs for barometric Vertical Navigation (VNAV) approach procedures.

1.2 Guidance Applicability

This document is applicable to Headquarters (HQ) and Regional Aircraft Certification personnel, including delegates and industry.

1.3 Description of Changes

This Advisory Circular (AC) replaces Aircraft Certification Policy Letter (ACPL) 57 Issue 1 dated 2001-12-07 and contains a fundamental change with respect to how temperature compensation is to be applied.  Whereas ACPL 57 called for application of temperature compensation for all temperatures, this AC requires temperature compensation only when below International Standard Atmosphere (ISA) conditions exist at the destination aeroport.  The requirement for above ISA compensation has been suspended pending resolution of operational considerations, promulgation of an operational requirement and provision of operational guidance and training material.

Other changes reflected in this AC include a clarification that temperature compensation is to be applied to the Minimum Descent Altitude/ Decision Altitude of an approach procedure.  This is consistent with the current cold temperature compensation procedure in Nav Canada Air Pilot, Canada Air Pilot General Pages (CAP GEN).  In addition, the International Civil Aviation Organization (ICAO) “accurate” method has been included as an acceptable means of temperature compensation.

1.4 Termination

This document does not have a terminating action.

2.0 References

2.1 Reference Documents

It is intended that the following reference materials be used in conjunction with this document:

  1. Chapter 523 of the Airworthiness Manual (AWM)—Normal, Utility, Aerobatic and Commuter Category Aeroplanes;
  2. Chapter 525 of the AWM—Transport Category Aeroplanes;
  3. Chapter 527 of the AWM—Normal Category Rotorcraft;
  4. Chapter 529 of the AWM—Transport Category Rotorcraft;
  5. Transport Canada Publication (TP) 308—Criteria For the Development of Instrument Procedures;
  6. TP 14371—Aeronautical Information Manual;
  7. Nav Canada Publication Number CAPGEN—Canada Air Pilot, General Pages;
  8. Federal Aviation Administration Advisory Circular (FAA AC) 20-129—Airworthiness Approval of Vertical Navigation (VNAV) Systems for Use in the U.S. National Airspace (NAS) and Alaska;
  9. International Civil Aviation Organization (ICAO) Document 8168-OPS/611—Procedures for Air Navigation Services, Aircraft Operations, Volume I, (as updated and expanded at ICAO Obstacle Clearance Panel number 12 in July 1999); and
  10. RTCA Inc. Document DO-236B—Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation.

2.2 Cancelled Documents

As of the effective date of this document, the following document is cancelled: 

  1. Aircraft Certification Policy Letter (ACPL) 57 Issue 1 dated 2001-12-07—Flight Management System (FMS) Barometric Vertical Navigation (VNAV) Temperature Compensation.

2.3 Definitions

The following definitions and terms are used in this document:

  1. FAF means “Final Approach Fix”.
  2. IAF means “Initial Approach Fix”.
  3. MAP means “Missed Approach Point”.
  4. MAHP means ”Missed Approach Holding Point”.
  5. MDA means “Minimum Descent Altitude”.

3.0 Background

Barometric altimeters are calibrated to indicate true altitude only under International Standard Atmosphere (ISA) conditions of temperature and sea level pressure.  In cases where the temperature is higher than ISA, the true altitude will be higher than the altitude indicated by the altimeter. Conversely, when the temperature is lower than ISA, the true altitude will be lower than indicated.

The effects of cold temperature on published instrument approach procedure altitudes have long been recognized in Canada.  Although non-precision approach procedures in Canada are designed using Transport Canada TP 308, Criteria for the Development of Instrument Procedures, these criteria that do not consider off standard temperatures, Transport Canada has provided a procedure to manually correct minimum IFR altitudes specified in approach procedures when the approach aeroport temperature is below 0°C.  The procedure is described in Nav Canada, Canada Air Pilot General Pages (CAP GEN).  However, the procedure is quite cumbersome requiring a pilot to use a table to manually calculate the correction for each of the minimum IFR altitudes of the approach procedure.

Flight Management Systems that provide VNAV capability for non-precision approaches may incorporate both extended constant angle vertical path for the final approach course as well as providing point-to-point vertical navigation between waypoints in the approach procedure.  With the extended constant angle vertical path feature it is not necessary to follow the vertical profile defined by the minimum IFR altitudes of the approach procedure waypoints:  the constant angle vertical path can be intercepted and flown like an ILS.  This is a feature that is being advocated by various aviation groups in the interest of prevention of Controlled Flight Into Terrain (CFIT).  These two baro VNAV descent profiles are illustrated in Figure 1.

Figure 1. FMS VNAV vertical approach

Both of the illustrated FMS vertical paths are defined by barometric altimetry.  Consequently the vertical paths would only be at the published procedure altitudes when ISA conditions exist.  In below ISA conditions, the constant angle vertical path would be at a shallower angle than for ISA conditions.  Likewise the vertical profile defined by the waypoint altitudes would also be lower.  In such cases expected terrain and obstacle clearance during approaches would not be maintained if the aircraft were flown at minimum IFR altitudes, nor would pilots be made aware of this condition. Clearly there is a safety consideration to be addressed for FMS baro VNAV terrain and obstacle clearance margin.

The need for an FMS to provide a capability to correct the published approach altitudes for below ISA temperature has been recognized and discussed by various groups including ICAO Obstacle Clearance Panel (OCP), and Special Committee SC-181 Working Group for RTCA DO-236B.

As part of the ICAO CFIT initiative OCP is considering the design and presentation of all types of non-precision approach procedures, in particular the need to take into account the stabilized approach technique and to provide a defined vertical profile to be flown for a minimum 5% gradient (3° glide path).  Recognizing the safety concern with respect to terrain and obstacle clearance below ISA conditions, this working group has introduced the concept of procedural altitudes to address stabilized approaches.

SC-181 Working Group has concluded its mandate with the publication of RTCA DO-236B.  While this document recognizes that not all states are agreed upon all the elements of temperature compensation, its Appendix H, Temperature Compensation Information, presents recommendations for implementation of below ISA temperature compensation.  Application of temperature compensation to the final approach vertical path angle as well as the waypoint altitudes associated with approach waypoints is addressed.

The information in the following paragraphs is consistent with the criteria included in RTCA Inc. document DO-236B with respect to below ISA conditions.

4.0 New or Updated FMS designs

Where Issue 1 of ACPL 57 called for “all temperature” compensation that is for both above and below ISA this Advisory Circular is revised to require that the final approach course vertical path angle and the waypoint altitudes associated with the approach procedure be compensated for below ISA conditions only.  While off-standard temperature conditions result in indicated altitudes different than true, Transport Canada is especially concerned with compensating altitudes in below ISA temperature conditions.  Transport Canada considers that Flight Management Systems that do not include a means to compensate below ISA conditions create an unacceptable loss of terrain and obstacle clearance when conducting VNAV approach operations.

4.1 New/Updated FMS Designs Incorporating Barometric VNAV Approach Capability

New or updated FMS designs shall provide a means for an aircraft to fly the true vertical path angle for final approach segment, as defined in the resident navigation database, in below ISA temperature conditions. The FMS equipment shall also provide the capability to temperature compensate all waypoints from the Initial Approach Fix to the Missed Approach Holding Point (known as the Missed Approach Holding Waypoint for RNAV procedures) inclusive, as coded in the navigation database.  The FMS shall also provide a means for determining a temperature compensated Minimum Descent Altitude (MDA)/Decision Altitude (DA), when the MDA/DA is entered by the pilot.

Temperature compensation may be applied for airport temperatures “below ISA” or “below 0°C”.  The latter has been included to be consistent with the existing Nav Canada operational procedures as described in CAP GEN.

4.2 Enabling Temperature Compensation

Transport Canada recognizes that some ICAO States do not require application of temperature compensation.  While Transport Canada Aircraft Certification requires that new or updated FMS include the capability to temperature compensate constant angle vertical path and minimum IFR approach altitudes for below ISA conditions, employment of this capability shall not be mandatory nor preclude the FMS from providing a vertical path or barometric VNAV descent profile if temperature compensation is not applied.

4.3 Prevention of Incorrect Temperature Selection

It is vitally important that the vertical path and minimum IFR altitudes are compensated for below ISA temperatures correctly. For example, an incorrectly entered sign for temperature in below ISA conditions would lower the vertical path and indicated waypoint altitudes when in fact they should be raised.

Where an FMS design requires manual input of temperature to enable temperature compensation the FMS shall provide a means for the pilot to confirm the intended input. FMS temperature compensation algorithms must provide indications for pilots to interpret how the vertical path and altitudes have been corrected.

4.4 Manually Entered Waypoint Altitudes

FMS designs usually include the ability to enter or edit waypoint altitudes.  This ability allows pilots to enter altitudes assigned by air traffic control (ATC). Normally, ATC expects that assigned altitudes will not be temperature compensated. For this reason, it shall be possible for waypoint altitudes to be manually entered or edited without automatically applying temperature compensation.

4.5 Annunciation and Indication

Temperature compensated altitudes displayed by the FMS shall be clearly differentiated from uncompensated altitudes.  In addition the altitudes being used for FMS guidance, compensated or uncompensated, shall be clearly annunciated.

4.6 Means of Temperature Correction for Barometric Altitude

The basis for correcting altitudes for off-standard temperature has been presented in ICAO PANS OPS Doc 8168.  Two correction methods are presented in this document, the “Simplified Method” as described in section 4.7 and “Accurate Method” as described in section 4.8.  Both methods are acceptable to Transport Canada and are summarized herein.  Note that the parameter definitions are specific to each method, in keeping with presentation in ICAO PAN OPS Doc 8168.  These methods are also reflected in section RAC 9.17.1 of Transport Canada publication (TP) 14371 – Aeronautical Information Manual.

4.7 Simplified Method

The simplified method provides a correction that is within 5% of the accurate method for airfield elevations up to 10,000 ft (3,000 m) and for heights up to 5,000 ft (1,500 m) above that airfield elevation.

Correction = H * (( 15 - t0 ) / (273 + t0 – 0.5 * L0  * ( H + Hss   )))
(See note below)

Where:

 

 

 

 

 

H

=

Minimum height above the altimeter setting source (setting source is normally the aerodrome unless otherwise specified)

ft

 

t0

=

taerodrome + L0 * Haerodrome which is the aerodrome (or specified temperature reporting point) temperature adjusted to sea level

°C

 

L0

=

Standard Lapse Rate (0.0019812 °C/ft or 0.0065 °C/m)

°C/ft

 

Hss

=

Altimeter setting source elevation

ft

 

taerodrome

=

Aerodrome (or specified temperature reporting point) temperature

°C

 

Haerodrome

=

Aerodrome (or specified temperature reporting point) elevation

ft

Table 1.  Simplified Method For Temperature Correction

Note: Correction values in this formula are to be added to “published minimum Instrument Flight Rules (IFR) altitudes”.

4.8 Accurate Method

The accurate method is more complex than the simplified method in that an iterative method must be used to determine the altitude correction.  This method is valid to 36,000 ft (11000 m).

Correction = ?hPaircraft - ?hGaircraft = (-?Tstd/L0) * Ln[(1 + L0 * ?hPaircraft(T0 +L0*hPaerodrome)]
(See note (a) below)

Where:

 

 

 

 

 

?hPaircraft

=

Aircraft height above aerodrome (pressure)

ft

 

?hGaircraft

=

Aircraft height above aerodrome (geopotential)

ft

 

?Tstd

=

Temperature deviation from the ISA temperature

°K

 

Lo

=

Standard temperature lapse rate with pressure
altitude in the first layer (sea level to tropopause)
of the ISA.
(See note (b) below)

 

°K/ft

 

To

=

Standard temperature at sea level

°K

 

hPaerodrome

=

Aerodrome height (pressure)

ft

Table 2.  Accurate Method For Temperature Correction

Note:

  1. Correction values in this formula are to be added to “published minimum Instrument Flight Rules (IFR) altitudes”.
  2. The lapse rate is defined as a negative number for this equation whereas it was defined as a positive number in the simplified equation.  This presentation has been kept consistent with the presentation of these equations in ICAO PANS OPS.

5.0 Installation of Existing FMS without temperature compensation

While implementation of temperature compensation procedures is an operational responsibility, Transport Canada Aircraft Certification has a responsibility to ensure that the end users are aware of the limitations of installed systems. Transport Canada recognizes that FMS installations exist which include a barometric VNAV approach capability, but do not include a temperature compensation function. For these installations, the following note shall be included in the Aircraft Flight Manual (AFM) “Limitations” section for the FMS.

"Barometric VNAV guidance during approach including the approach transition, final approach segment, and the missed approach procedure is not temperature compensated. Unless a temperature limitation is reflected on the approach chart, operating at uncompensated minimum IFR altitudes will not provide expected terrain and obstacle clearance for temperatures below ISA”

6.0 Headquarters Contact

For more information please contact:
Policy Standards Coordinator (AARDH/P)
Phone:              (613) 990-5742
Facsimile:         (613) 996-9178
E-mail:              mailto:AARDH-P@tc.gc.ca

Original signed by:

Gilles Morin
Chief, Regulatory Standards
Aircraft Certification Branch