Revisiting CFIT (Control flight into terrain)

 

By Philippe Burcier

 

 

1. INTRODUCTION

 

Control flight into terrain (CFIT) accidents have been and still are the major cause of aviation hull losses and fatalities

Since the introduction of the GPWS, the number of CFIT accidents has decreased significantly but we have not seen any noticeable reduction in numbers for 5 years.

Moreover, 1998 has started very badly with 4 CFIT accidents during the first 6 months.

How can these figures be explained and what would be the next step to make a positive improvement in the curve of CFIT accidents?

 

From December 1995 to July 1998:

Ten CFIT accidents occurred on jet commercial aircraft that were all equipped with GPWS.

One accident due to no, slow or wrong pilot response to GPWS alert occurred.

Two GPWS warnings came too late.

Six accidents occurred during phase where the warnings were desensitized.

Two occurred during non precision approaches ( 27 from 1983).

Four occurred during initial approaches.

CFIT accidents by type of instrument approach (July 1988 – July 1998)

 

From these statistics, we can draw the following conclusions:

 

The GPWS has globally reduced the rate of CFIT accidents. It has been and remains a very effective system but does not cover all CFIT risks and brings few results in approach and landing phases.

Solutions to the CFIT risks have to be found in new flight crew procedures and other technical improvements.

 

Let see the 3 following subjects:

 

Approach charts

Airline procedure

Aircraft equipment.

2. APPROACH CHARTS

 

Very few modifications have been made to these charts in the last 20 years. It is now recognized that a better design of some procedures and a new display of the published approach and departure charts may decrease the CFIT risks in these flight phases.

 

In particular, step down approaches have been a main factor of CFIT accidents. Approach procedures should be designed to allow a minimum number of step down during initial approach and a constant slope from the Final approach Fix to the MDA/MDH.

 

Let see now the ILS Rwy 6L chart. This chart can be used for ILS or LOC only approach.

AGANA (GUAM) Rwy 6L

CURRENT ILS and LOC ONLY APPROACH

 

 

* ILS slope is 3°, LOC only

Slope is 3.3°.

* “LOC ONLY” profile difficult

to extract from normal ILS

approach profile.

 

* DME info is not clear enough.

 

* LOC ONLY has 4 steps

requiring action on the FCU

with chance of error.

* Aircraft will be high if

MDA is flown to the MAP.

Pitch down will be required.

The NTSB, overseeing the crash investigation of the Guam accident, concluded that the LOC ONLY approach to Agana 6L demands precise and well timed actions.

This is confirmed by one senior airline captain reporting that the LOC ONLY approach in AGANA requires a thorough briefing, a full understanding of distances and altitudes and close coordination. If conditions deteriorate, he said that “ the problems facing the crew are enormous.”

This is especially true if the crew is unsure to fly a serviceable glide slope until late in the approach

 

PROPOSED MODIFICATIONS:

At least a DME-Altitude table to show a 3.3° glide slope from the FAF.

DME distance shown as – D, D 0.0, +D for clarity.

The DME being not collocated with the ILS, clear information on approach charts is vital.

Since August 98, Jeppesen have edited some new approach charts with constant flight path from the FAF down to the VDP (visual descent point).

This will be done to comply with the recommendation, now well accepted in the aviation community, to replace step-down approaches by constant slope of approximately 3 degrees.

Airbus Industrie has promoted this type of non precision approach for several years.

 

OPERATIONAL PROCEDURES:

Some procedure improvements are considered to be efficient to avoid CFIT accidents.

Let’s quote:

1. Stabilized trajectory from the FAF to the MDA/MDH.

 

Aircraft should be in landing configuration and approach speed when reaching the FAF.

The final segment should be flown on a constant slope of approximately 3 degrees.

When the aircraft is equipped with an FPA mode, it should be used for non precision approaches flown in selected modes.

 

AUTO CALLOUT

Airline procedures should define specific auto callout ( on Airbus aircraft, this is done using the FWC pin program).

A radio height awareness procedure might also be defined.

Rigorous altimeter setting and cross-checking should be defined in the airline procedures, particularly when the destination airport is located in countries using different units (millibars or inch of mercury associated with meters or feet) .

The FMS programming should be reduced to minimum during departure and approach.

Sterile cockpit during takeoff and landing phases should be envisaged.

 

TECHNICAL IMPROVEMENTS

EGPWS

The major technical improvement will come from the EGPWS (Allied Signal). This new system provides a cockpit terrain display and advanced warnings of CFIT risks.

This system will be very efficient when the terrain database becomes sufficiently accurate in all parts of the world. Allied Signal has launched a vast program to reduce the terrain database discrepancies. This will be effective with the next issue of the database.

The EGPWS is currently certified on A320 family with QNH selection.

During the first quarter of 1999, the system will be certified for QNH and QFE options.

EGPWS will be basically installed on all fly by wire commencing next year.

GCAS

Another system called GCAS (Dassault / Sextant) is currently developed and will provide similar functions.

 

CONCLUSION

With 4 accidents during the first 6 months of 1998, CFIT has remained the first cause of aviation accidents responsible of most of the aviation fatalities.

The CFIT problem needs to be re-addressed and a campaign to be launched at all levels of the aviation community.

Technical equipment such as the EGPWS (later on GCAS) will reduce the risk of CFIT, but the immediate improvement will remain within airline’s hand.

From the top management to the flight crews, each member of the aviation community has to be aware of the causes of CFIT accident and propose adapted procedures and techniques.

Flight crews should always remind themselves that they are the last line of defense against CFIT.