2nd International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR'2013) March 17-18, 2013 Dubai (UAE)
Development of an Algorithm for Aircraft Tail
Strike Avoidance during Takeoff and Landing
Harjeev Singh Anand1, Cibi Vishnu Chinnasamy 2, J. Darshan Kumar 3 and V.R. Sanal Kumar 4
crash land or overshoot depending on whether the aircraft is
aligned with the runway or not, and also the vertical distance
between the aircraft and the ground determines a perfect touch
down at correct marker position. Also if the main wheels are
not in an axial plane with the ground, non-uniform wheels
touchdown occurs leading to one sided landing, which might
cause tire burst due to excess stress on one wheel. Thus all
these factors must be properly calibrated for an easy
touchdown.
A tail strike occurs if the tail of an aircraft touches the
runway during takeoff or landing. It can occur with any type
of aircraft, although long aircraft may be more prone to tail
strike, because tail strike occurrence is directly related to pitch
attitude versus aircraft geometry and main landing gear status.
Although many tail strikes occur on takeoff, most occur on
landing. Tail strikes can result in significant structural damage
to the aircraft and, cost operators millions of dollars in repairs
and lost revenue, and therefore jeopardize the safety of the
flight and lead to considerable maintenance action. In the most
extreme scenario, a tail strike can cause pressure bulkhead
failure, which can ultimately lead to structural failure;
however, long shallow scratches that are not repaired correctly
can also result in increased risks. Tail strikes are reported
often due to human error. Fig.1 demonstrates the planned tail
strike of Boeing 777.
Abstract—Airlines industry often reported aircraft tail strike
accidents due to the Pilot and/or instrument errors at the time of
takeoff and landing. In this paper an attempt has been made to
develop an algorithm for aircraft tail strike avoidance during the
takeoff and landing with the help of laser proximity sensors. After
considering the response time of the aircraft rotation and other
movements upon onboard commands, the laser proximity sensors
with a response time of 1 µs and a resolution of 0.5 µm is
recommended for commissioning it near the tail of the aircraft for
data acquisition and interfacing with the algorithm. Using this
algorithm, Pilot will be getting an online feedback for setting the
correct angle of attack and/or pitch up/down angle with desired
takeoff/touchdown velocity of the Aircraft without any tail strike by
retaining a safe tail clearance height. This algorithm can also be used
for autopilot system.
Keywords— Aircraft tail strike, Autopilot system, Landing,
Proximity Sensors, Takeoff.
I. INTRODUCTION
HE landing and take-off are the most difficult part of a
flight journey; especially during the unexpected
conditions such as rainstorm and foggy situations the
landing as well as the take-off of an aircraft become a very
perilous task. The effect of proper Take-off speed is important
when runway lengths and landing distances are critical. The
take-off speeds specified in the aircraft flight handbook is
generally the minimum safe speeds at which the aircraft can
be landed. Often during foggy situations, where the visibility
is limited, the pilot's judgment on take-off and landing
parameters is very vital. If there is an error in the judgment,
then there is a chance of tail-strike to occur. The effect of
touch down velocity and the vehicle orientation are also very
important; and if the pilot commits any mistake while landing
or if his judgement is erroneous then the flight may either
T
Harjeev Singh1 Anand, and Cibi Vishnu are with Chinnasamy2 are
undergraduate students with Department of Electronics and Instrumentation
Engineering, Kumaraguru College of Technology, Coimbatore–641049, Tamil
Nadu, India; email id: harjeev304@gmail.com, cibivishnu7@gmail.com
J.Darshan Kumar3 is currently Assistant Professor, and with Department of
Aeronautical Engineering, Kumaraguru College of Technology, Coimbatore –
641 049, Tamil Nadu, India; email id: darshankct@yahoo.in
V.R.Sanal Kumar4 is currently Professor and Aerospace Scientist, and with
Aeronautical Engineering, Kumaraguru College of Technology, Coimbatore –
641 049, Tamil Nadu, India; Corresponding Author, Phone:+91-9388679565;
email id: vr_sanalkumar@yahoo.co.in
Fig. 1 Demonstration of a planned tail strike of Boeing 777.
Literature review reveals that about 25% of reported tail
strikes occur at takeoff and 65% at landing and 10% not
properly cataloged [1]. Tail strikes on landing generally cause
more damage than takeoff tail strikes because the tail may
strike the runway before the main gear, damaging the aft
pressure bulkhead. Admittedly, many operational and human
factors are involved in tail strikes at takeoff. Analysis of in238
2nd International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR'2013) March 17-18, 2013 Dubai (UAE)
service events highlighted that the following factors may
reduce, when combined, the tail clearance margin (i.e.
distance between the aircraft tail and the ground) at takeoff,
such as (i) early rotation, (ii) rotation technique, (iii) thrust /
weight ratio, (iv) slats / flaps configuration, (v) erroneous CG
position and trim setting, (vi) crosswind, (vii) shock absorber
oleo inflation.
Published reports further corroborated that more tail strikes
occur on landing than on takeoff. It is reported in 2004 that
82% of 737-400 tail strikes occurred on landings and 737-400
does not have tail skid protection for landing therefore has
higher damage. Also reported that 70% of 737-800 tail strikes
occurred on landings and 737-800/900 tail skid does not
protect the aircraft body for landing. However, 737-800/900
have adequate aft body landing clearance. It can be seen from
the open reports that 1994-1995 was the tail strike peak period
with all Boeing models due to increased deliveries and/or new
pilots [2].
A case on hand reveals that one of the passenger aircraft at
Melbourne Airport planned a reduced-power takeoff but it
was unsuccessful [5]. In an attempt to resolve this problem the
Pilot made an attempt to rotate the aircraft, but it did not
respond immediately with a nose-up pitch. The pilot again
made an attempt for a greater nose-up command, while doing
so aircraft was raised and the tail made contact with the
runway surface but failed to climb. Immediately the Pilot
selected Takeoff/Go-around switch (TO/GA) on the thrust
levers, the engines responded immediately, and the aircraft
commenced a climb [5]. This incident has created a serious
concern for retaining a safer distance between ground surface
and aircraft tail during takeoff.
On Boeing aircraft TOGA modes are selected by a separate
switch near the throttle levers, but on Airbus aircraft it is
activated by pushing the thrust levers fully forward to the
TOGA detent. A detent is a device used to mechanically resist
or arrest the rotation of a wheel, axle, or spindle. The response
time of such device is very important at the time of
emergency. Such a device can be anything ranging from a
simple metal pin to a machine. Though the existing takeoff
system is good in its own right there are many limitations.
Note that one of the important factors at the takeoff is the
response time of the instruments. In addition to the pilot and
instrument errors low response time of the instruments can
lead to aircraft failures at takeoff and landing. Fig.4 is
demonstrating the aircraft takeoff and initial climb at various
time intervals.
Fig. 2 Reported tail strike of a passenger plane [3]-[4].
The Fig.2 shows the tail strike of a passenger plane reported
in the open literature. Of late, aircraft failure analysis report
reveals that certain accidents are reported due to the flight
instrument malfunction —discoverable by the pilots, but
subtle in its impact on the automated flight controls in use [34]. Literature review further reveals that takeoff difficulties
are experienced in many passenger aircraft. Fig.3 shows the
typical takeoff tail clearance profile.
Fig. 4 Demonstrating the aircraft takeoff and initial climb.
Pilots are used to calculating takeoff speeds and, therefore,
understand the operational significance of V 1 , the decision
speed, which is the maximum speed at which aircraft is still
possible to reject the takeoff and stop the aircraft within the
runway limits, V R , the rotation Speed and V 2 ,the takeoff
safety speed. However, report reveals that the Pilots are
slightly less familiar with the definitions of V MU (Velocity of
Minimum Unstick), V MCG (Velocity of Minimum Control on
Ground), and V MCA (Velocity of Minimum Control in the
Air). Takeoff speeds are a safety key element for takeoff, and
enable pilot situational awareness and decision-making in this
very dynamic situation. The use of erroneous takeoff speeds
can lead to tail strikes, high-speed rejected takeoffs or initial
climb with degraded performance.
As highlighted by the Capt. Ray Craig during the Boeing
tail strike briefing [6] the typical takeoff risk factors reported
are (i) mis-trimmed stabilizer, (ii) improper rotation
Fig. 3 Typical takeoff tail clearance profile.
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2nd International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR'2013) March 17-18, 2013 Dubai (UAE)
techniques, (iii) improper use of the flight director, (iv)
rotation prior to Vr, (v) excessive initial pitch attitude, (vi)
heavy derate/flight control abuse during gusty/crosswind
conditions. The mis-trimmed stabilizer usually results from
using erroneous data like wrong weights and incorrect center
of gravity (CG). Also the nose up mis-trim can present
problems. Note that according to Boeing the normal
recommended rotation rate is 2 to 3 dps. However nose up
mis-trim can rotate 5 dps or more. Ray Craig [6] further
reveals that aircraft may try to fly off runway without any
pilot input due to mis-trimmed stabilizer. The author also
reported that crosswind landings may increase the tail strike
risk, especially in gusty conditions. To stay on glide path at
high ground speeds, descent rates of 700 to 900 feet are
required. Cross controlling prior to touch down, reduces lift,
increases drag, and may increase rate of descent. Combined
effects of high closure rate, shifting winds plus turbulence,
can increase tail strikes. Go-around initiated during flare and
after a bounced landing, can also cause tail strikes. Ray Craig
also summarizes that more tail strikes occur on landing than
on takeoff. The factors that increase the probability of a tail
strike during landing are demonstrated in Fig.5.
II. METHODOLOGY
The selection of a suitable proximity sensor is critical for
the proposed aircraft tail strike avoidance system.
A sensor (also called detector) is a converter that measures
a physical quantity and converts it into a signal which can be
read by an observer or by an (today mostly electronic)
instrument. A good sensor obeys the following rules, such as,
(i) sensitive to only the measured property, (ii) insensitive to
any other property likely to be encountered in its application,
(iii) does not influence the measured property. Ideal sensors
are designed to be linear or linear to some simple
mathematical function of the measurement, typically
logarithmic. The output signal of such a sensor is linearly
proportional to the value or simple function of the measured
property. Often in a digital display, the least significant digit
will fluctuate, indicating that changes of that magnitude are
only just resolved. The resolution is related to
the precision with which the measurement is made.
A. Selection of Laser Proximity Sensor
A sensor with high response time and high resolution and
high dynamic characteristic is ideal for distance measurement
between the ground and the tail of aircraft. This sensor is
integrated with the autopilot and auto throttle system. The
primary aim of using the sensor is to prevent tail strike by
continuously monitoring the tail-ground distance and maintain
a safer tail clearance height through a feedback governing
system. After considering the response time of the aircraft
rotation and other movements, the required characteristics of
the proposed laser proximity sensor are; (i) time response: less
than or equal to 1 µs, (ii) resolution: 0.5 µm, (iii) range: 0.5 to
4000 mm, (iv) bandwidth: 100 kHz.
B. Deployment and Commissioning Challenges
In order to minimize the tail strike accidents, a laser
proximity sensor could be implemented near the tail of the
aircraft (see Fig.7). The suitably calibrated sensor measures
the actual distance between the ground and the tail of aircraft
whatever maybe the orientation of the sensor. For long
airplanes, which are more prone to tail strikes, the pilot can
know the exact distance of the tail from ground and if that
distance is very small, a warning alarm is sent to the pilot.
This allows the pilot/autopilot to initiate action for tail strike
avoidance. Note that while landing and takeoff, distance of the
aircraft tail from the ground could be monitored by this
calibrated sensor and action will be initiated to prevent the tail
strike.
Fig.5 Operational and human factors involved in tail strikes at
landing.
Fig.6 Demonstrating the unstabilized approach of an aircraft.
An unstabilized approach is the biggest single cause of tail
strike during landing. Fig.6 is demonstrating the unstabilized
approach of an aircraft with 3 degree glide slope. Tail strikes are
costly but can be prevented with proper technology and
training. Failure analysis reports of various committees reveal
that airlines have to go for a routine maintenance procedure
for fault-tolerant disk arrays that ensures the entire user data is
protected correctly by checking data consistency for both
parity and mirrored data, while checking the media integrity
as well. To overcome all these issues in this paper we propose
a laser proximity sensor with automatic feedback system for
correcting the errors and prohibiting the tail strike.
Fig.7 Showing the location of the proposed laser proximity sensor.
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2nd International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR'2013) March 17-18, 2013 Dubai (UAE)
C. Development of an Algorithm
fool proof [1-10]. As an alternative or as a supplement in this
paper an attempt has been made to develop an algorithm for
aircraft tail strike avoidance during the takeoff and landing
with the help of laser proximity sensors.
The following factors, as succinctly reported in the
AIRBUS flight operation briefing notes, are often observed
when analyzing takeoffs in which takeoff speeds were not
respected. Two cases can be observed; viz., (i) error in takeoff
speed computation, (ii) error in takeoff speed utilization. In
the case of an error in takeoff speed computation; data, issued
from a computerized system, is rarely challenged. However,
incorrect inputs may occur, and could result in inadequate
takeoff speeds values. In takeoff speed calculations, Zero Fuel
Weight (ZFW) is sometimes mistaken for Gross Weight
(GW). This is particularly true when a last minute change
occurs in cargo loading, or when time pressure and workload
are high. Therefore, calculated speeds will be much lower
than expected, and will lead to: Tail strikes, “heavy aircraft”
sensation, and high-speed rejected takeoffs. The computation
of ‘h’ can be done using Pythagoras theorem subjected to the
ground level and the online computation ‘h’ depends on the
various takeoff / landing parameters of the aircraft. Note that
takeoff speeds calculations are based on specific
configurations. Any change in the parameters of these
configurations will invalidate takeoff speeds. Examples of
such parameters include a runway change, a wet runway that
becomes contaminated, or a takeoff from an intersection.
The commissioning of laser proximity sensors with close
loop guidance system in an aircraft as proposed through this
paper, as shown in Fig.7, can definitely avoid human errors
causing the tail strike. The algorithm also considers the total
weight of the aircraft at takeoff, local density and lift
coefficient with respect to angle of attack. Therefore
computation of “h” will be more accurate. Note that rotating
the aircraft at the appropriate time, proper rate and correct
takeoff speed can avoid the tail strike. Rotating early means
less lift and less aft tail clearance. Whenever the aircraft tail
exceeds the safe tail clearance the laser proximity sensor will
swiftly detect and the algorithm will activate for pitch
corrections in accordance with the correct takeoff speed.
Rotating at the proper rate is very important for avoiding tail
strike and one should not rotate the aircraft at an excessive
rate or to an excessive attitude. When a last minute-change
occurs, takeoff speeds are sometimes modified and that will
be automatically updated by the algorithm. It is recommended
to get the correct takeoff weight for the aircraft for a better
performance prediction and safe control.
An effort has been taken for developing an algorithm after
considering all the input variables including aerodynamics and
local environmental data causing the tail strike. It may be
noted that the laser proximity sensors are having bearing on
the human errors causing the tail strike owing to the fact that
the sensor will be continuously monitoring and maintaining
the safe tail clearance height, A, with the help of automated
feedback system. The proposed sensor is evidently having fast
response for correcting the aircraft rotation and climb rate to
avoiding the tail strike. Algorithm will also evaluate the safe
taxing time / distance; takeoff velocity, rate of climb, based on
the total weight of the aircraft, local air density, lift coefficient
and frontal area corresponding to the angle of attack set by the
Pilot. The tail height from the ground, h, will be continuously
computing with the help of a subroutine specifically
developed using the moving boundary computation based on
the landing or takeoff ground / run way surface mapping. The
sequence of operations is shown in the flow chart in the
subsequent session D.
D. Flow chart
IV.
CONCLUDING REMARKS
The takeoff speed calculation errors leading to tail strike are
often due to a combination of two factors; viz., (i) error in
parameter entry, (ii) poor crosschecks by other crewmember.
Therefore suitable prevention strategies should be developed
to ensure efficient crosschecks, particularly after last-minute
changes such as, runway change, load sheet modifications
III. RESULTS AND DISCUSSION
Although many measures are taken in the airlines industry
for the aircraft tail strike avoidance, none of these methods are
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2nd International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR'2013) March 17-18, 2013 Dubai (UAE)
etc.. Therefore instead of keeping a safe takeoff weight
margin, it is recommended to keep the aircraft landing gears
over the load cells at the airport for an accurate estimation of
the total weight at each and every takeoff owing to the fact
that aircraft total takeoff weight would be varied due to fuel
weight variations, passenger/cargo weight variations etc.. The
commissioning of an additional lucrative feedback control
system using laser proximity sensors, for detecting and
ensuring the minimum tail clearance in the event of
computational / human / instrument errors, can definitely
avoid tail strike. It is well known that an unstabilized
approach is the biggest single cause of tail strike during
landing. This can be reduced by commissioning multiple
proximity sensors at the proper locations of the aircraft for
data acquisition for the close loop guidance system for
invoking the stability of the aircraft. We concluded that the
proposed laser proximity sensor interface with the given
algorithm is a viable option for the tail strike avoidance of
aircraft during takeoff and landing.
ACKNOWLEDGMENT
The authors would like to thank the college Management
and Mr.Shankar Vanavarayar, Joint Correspondent of
Kumaraguru College of Technology, Coimbatore – 641 049,
Tamil Nadu, India for their extensive support of this research
work.
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