HARD LANDINGS - Helping pilots better handle the airplane during landing

 

SOURCE: NTSB and FAA

Advisory Circular 120-71 "Standard Operating Procedures for Flight Deck Crew Members" and Flight Standards Information Bulletins for Air Transport (FSATs) 00-08 and 00-12.

 

World Intellectual Property Organization (WIPO)

HARD LANDING REPORT BASED ON SINK RATE ALGORITHM

Issue: High sink rate awareness during landing.

Foresight bounce recognition and recovery

Hard or heavy landings are significant high load events that may adversely impact airframe structural integrity. Such landings may result in damage that affects the ability of the aircraft to fly safely. When this happens, repairs must be performed prior to flying the aircraft again.

The inspection process that is required to assess the potential for damage due to a suspected hard landing event is undesirably time consuming.

Studies showed that up to 90% of pilot-initiated hard landing inspections resulted in no finding of damage.

The results of performing unnecessary inspections include undesirably increased labor costs and lost revenues due to the down time of the aircraft.

Study case

Strong pitch up after the second hard touch-down and strong nose-down pitch forces.

Boeing defines hard landings that exceed 12.3 feet per second [fps] or that involve rapid derotation [lowering the nose wheel to the runway after the main gear touches down after the initial touchdown as severe.

1. ·         Pilot monitoring for high sink rates.
2. ·         Appropriate timing of the landing flare.
3. ·         The flare is based on gross weight, temperature and pressure.
4. ·         Airspeed trend vector is useful tool in determining when to begin to flare.
5. ·         Aural altitude calls and the radar altimeter.

You don't want that accident investigation final report writes down "the cause of the accident it was the pilot-flying inability to arrest the high rate of descent existing at 50 feet [ft] radio altitude."

"SINK RATE" aural alert from EGPWS [Enhanced Ground Proximity Warning System] it is your first and foremost calling for your attention.

The load factor provided by an air data inertial reference unit AD RU), which is not reliable due to the body-bending response in the fuselage at touchdown.

The ADIRU is located at the forward section of the fuselage and the load factor is mathematically translated to the airplane's center of gravity to determine the load factor value. Data analyses of actual landings from operators have shown that the load factor is an unreliable indicator of a hard landing event.

A false report of a hard landing can result in an unnecessary costly structural inspection and has the potential to delay dispatch of the airplane.

The indication of a hard landing as reported within an airplane condition monitoring function (ACIV1F) is based on data recorded from nose[1]mounted accelerometers, combined and recalculated to correct for the true location of the aircraft's center of gravity.

The sink rate algorithm comprises a second-order complementary filter

followed by a lag time noise reduction (i.e., smoothing) filter. The output main gear vertical sink rate takes into account the landing gear position with respect to the runway surface.

Activation of the sink rate computation occurs at some preset elevation (e.g., 200 feet) above ground level of the wheel carriage as determined by the radio or radar altimeter.

Monitoring continues until a predetermined time (e.g., 2 second) after the point of touchdown.

The sink rate algorithm disclosed herein has application for reporting hard

landings by aircraft of different types. The sink rate algorithm disclosed herein has been adapted for use with models 200 and 300 of the Boeing 777 aircraft.

The sink rate algorithm disclosed herein is based on a design that has been widely used in autopilots.

FIG. 1 shows the main components of a hard landing detection system in accordance with one embodiment of the invention. The ACMF 10 comprises a logic unit for performing the steps of a sink rate algorithm, such as the algorithm depicted in FIGS. 2A[1]2D. The sink rate algorithm outputs a main gear sink rate in response to the inputting of the following parameters: (1) radio altitude (in feet; + is up); (2) pitch attitude (in degrees; + is nose up); (3) body pitch rate (in deg/sec; + is nose up); (4) vertical speed (feet/min; + is up); and (5) vertical acceleration (g; + is up). The ACMF 10 receives radio altitude data from a radio altimeter 14, which is mounted on the airplane. The ACMF 10 receives data representing values of the other four parameters from an ADIRU 12. As will be explained in more detail later, the ACMF also receives data representing the current gross weight of the airplane from a flight management function (FMF) 16.

The sink rate output is smoothed with a quarter-second time constant lag filter to provide a clean, well-behaved estimate of the sink rate during flare and touchdown. This algorithm is currently being used within Flight Controls on the 777 because of its accuracy.

Using the vertical acceleration parameter to calculate sink rate has an advantage over using the vertical speed in that the vertical acceleration is not corrupted by ground effects as the airplane nears the ground.

Ground Proximity Warning System (GPWS)

Mode 4 – Terrain Clearance Not Sufficient (while in landing configuration). Mode 4A and 4B are active during cruise and approach, and Mode 4C is active during go-around. Mode 4A triggers “Too Low Terrain, Too Low Gear” when the landing gear is up, Mode 4B triggers “Too Low Terrain, Too Low Flaps” with flaps not in landing configuration (but landing gear down) and Mode 4C triggers with flaps not in landing configuration OR gear up: “Too Low Terrain”.

Mode 5 – Excessive Descent Below Glide Slope – triggered when the aircraft descends below the glideslope and the aural alert “Glideslope” is triggered.

Note: the above warnings, cautions and callouts differ depending on the aircraft type (and can even differ on the same type of aircraft when different systems are installed).

Enhanced GPWS

Enhanced GPWS (EGPWS) supplements Basic GPWS with a database of terrain and airports, and correlates this with the known position of the aircraft.

EGPWS (or “Predictive GPWS”) has a computer model of the aircraft performance and uses this to create a caution and warning envelope in front of the aircraft, including the ability of the aircraft to climb.

When the Predictive GPWS is operating normally the Basic GPWS Mode 2 (Excessive Terrain Closure Rate) is inhibited. If a failure is detected in the Predictive GPWS, or there is a significant discrepancy between detected rad alt height and the T2CAS altitude, Basic Mode 2 is re-enabled.

On Basic GPWS

Mode 2 – Excessive Terrain Closure Rate: Mode 2 takes into account gear and flap configuration. There are two types of Mode 2 alerts: Mode 2A (active during climb, cruise and initial approach) and Mode 2B (active during approach and 60 secs after takeoff). With landing gear up the warnings are “Terrain”, “Terrain Terrain” and “Pull Up”. With landing gear down, the “Terrain” caution is triggered.

Airbus A320/A330/A340 Predictive GPWS – Warnings and Cautions

The Airbus A320/330/340 aircraft utilize a Terrain Awareness Display (TAD) function which develops a caution and warning envelope in front of the aircraft. The TAD takes into consideration the aircraft’s altitude, nearby runways and the altitude of the runway, together with the aircraft’s speed and turn radius.

When the system detects a threat in the projected envelope it will trigger the relevant GPWS caution and warning callouts (aural alerts).

Adam B733 at Surabaya on Feb 21st 2007, hard landing.

Adam Air Boeing 737-300, registration PK-KKV performing flight KI-172 from Jakarta to Surabaya (Indonesia) with 148 passengers and 7 crew, was approaching Surabaya's runway 28 in thunderstorm rain, visibility 8000 meters. When the aircraft descended through 200 feet AGL the captain called the aircraft was too high and took control, subsequently the Ground Proximity System issued alerts "Pull Up!" and "Sink Rate!" The right hand main gear touched down outside the runway, about 4 meters off the right edge of the runway. The captain steered the aircraft back to the center line of the runway and brought it to a stop about 100 meters short of taxiway N3. Two passengers received minor injuries (backbone pain), the aircraft received substantial damage including a fractured/bent fuselage.