Landing Performance
Landing Performance
Let’s revise it from different angles.
Dispatch
Requirements:
One
of the many tasks of a dispatcher and the crew before a flight is to calculate
the landing numbers and for us who operate under part 135 of the FAR we are
allowed to use as much as 60 percent (80% DAAP if allowed by the Ops Specs) of
the most favorable runway in terms of wind direction, intensity, landing aids
and terrain. So here we go, from here on
is all fine print.
This 60
percent allowed is compared to the numbers given by the manufacturer which are
produced during flight tests and even thought the parameters are written under
part 23 and 25 of the FAR they are squeezed from the airplanes to get from them
every bit of what they are capable of.
We would possibly never be able to replicate them in our
operations.
For instance:
They are predicated on a speed that has no additives at 50 feet and around 6
feet per second at touchdown, which doesn’t sound very much until you see that
splattering the aircraft at 360 feet/minute would raise some eyebrows in the
back, then hold on since its full manual braking as if there is no tomorrow,
all of that in a pristine runway sometimes specially cleaned for the occasion.
From here on you are standing on a number that is not really useful, it is
there to comply with the regulations and will help the sales departments pitch
you places where you can land their planes.
Continuing with our flight, the dispatcher will
compute your Regulatory Landing Distance from the Effective Runway Length by
calculating 60 percent of it, then determine the Actual Landing Distance from
the AFM and compare the two numbers, this is the first step, various things can
affect this numbers, for example: NOTAMS can decrease the Landing Distance
Available and MEL items can increase the Actual Landing Distance. The second step if needed is done in case of a
wet runway where the Actual Landing Distance has to be factored by 1.67 and then
add an additional 15 percent for the wet penalty; this number has to be less than
the Effective Runway Length if the runway is contaminated the number has to be
smaller than the effective runway length too.
This is all math no flight tests whatsoever wet and contaminated numbers
are usually mathematically produced as they call them “RSC DEPTH” Runway
Surface Contamination in Terms of Equivalent Water Depth. In the manuals usually you can find notes
that will say this one way or the other.
Some will say: The data presented comes from analytical data and not
from flight tests.
As you see
the calculations for your dispatch begin with a flaw and then factors related
to the regulations which try to give some balance for the unusable
numbers. Those are the margins everybody
is dispatched with and those are the numbers that keep us flying every
day. Even though they are useful for
dispatch in the sense that will bring a margin they are all guesses at best.
Flight
Requirements:
Once
we are airborne the rules change, we are no longer required to factor in any
additions, we can use our factory given numbers to land, but remember they are
the factory numbers that are uniquely produced.
Before 2006 this statement was absolutely true and because of industries
concern triggered by the runway overrun of a Boeing 737 in Midway airport a
SAFO was issued, SAFO 06012 is the industries response to this nonsense of
giving airmen landing distances they will never be able to replicate. At the same time “TALPA ARC” was born, an
Aviation Rulemaking Committee to address Take Off and Landing Performance
Assessment; from it an “ARCAM” Runway Condition Assessment Matrix was
developed, the Matrix as you see below levels the field within pilot reports,
reported runway conditions and airport runway assessments. For example you may have a reported “GOOD”
Braking Action but your runway has “COMPACTED SNOW” so you will no longer use
“GOOD” as your reported braking action but “GOOD to MEDIUM” and so forth,
depending on the factors the manufacturer of the airplane you fly uses you can
use the Matrix to be consistent. This is a great effort from the industry to
bring truth to the cockpit so we can better take into account the factors.
Now
would be the turn of the aircraft manufacturers to provide us with useful
numbers regarding the operation. What
they usually give us is Sea Level, ISA, no wind, no engine reverse, Vref, Max
Braking and what life throws at us every day is a world apart specially in
regards to braking effort. Some manufacturers are already doing this change and
hopefully they will encourage other manufacturers to do so too. One of the main reasons not to provide
numbers other than Actual Landing Distance is that there is no means today
inside most cockpits to measure deceleration.
Since there is nothing accurate to relate to, there is a sea of
ambiguity of what each of us would consider enough braking. For aircrafts equipped with “AUTOBRAKES”
deceleration is given at fixed rates on request by the crew, then the numbers
are easier to produce since there is a parameter from which to start.
Let’s
see what this manufacturer says about a normal 3.3 Kts/second squared
deceleration. What the table shows is
amazingly close to what we usually see in normal operations. The numbers are as follows: For an aircraft
landing on a dry runway at its maximum landing weight the Actual Landing
Distance is 3542’ (Flight Test Data), for the same aircraft and conditions
landing with a fixed 3.3 Kts/s2 deceleration the landing distance increases to
6360’, if more braking effort is requested up to 5.8 Kts/s2 the landing
distance shortens to 4460’, that is really close to real life. And for the first case presented it’s an 80%
increase in Landing Distance from the flight test data. On the data given here you would add the 15%
increase from the SAFO and that would bring your 6360’ to 7314’ which will put
this airplane outside of the boundaries of many airfields that we commonly use
if a low braking effort is used. But in
reality we have the Actual Landing Distance and are required by the SAFO to add
15% to it, for the case above that would be a Landing Distance Required of 4047
feet which is a long shot from the real numbers.
On
this subject you see we are on our own since there is today no real way to come
to a useful number for any given flight in most of the airplanes we fly and
that the real numbers vary from 1.8 to 2 times from the flight test data on a
dry runway.
Now that we
have our dispatch and ready to go the ball is in our court. Interestingly enough the NTSB accident data
base has most of the accidents related to runway overruns as dispatched
correctly, and there is usually something to say about the crew doing this or
not doing that. The accident data base
from 1995 to 2008 puts landing accidents being 30% of the total accidents and
from this 97% are runway excursions. From the numbers we revised before we know
that there are things you can do to tilt the outcome in your favor. Here are
some of them: Do the math and put in the penalties to get a realistic figure,
for this task use your experience in the aircraft, fly a stabilized approach,
land where you intended to, go around if you don’t feel comfortable or if the
parameters drift outside of the SOPs and be vigilant of your tolerances and
comfort zone. If it doesn’t feel
right it probably isn’t.
Stabilized Approaches:
Here you have the common information given to us in ALAR seminars and we
will add penalties to the Landing Distance when not within the parameters.
Approach-and-landing Accident suggests that
"all flights must be stabilized by 1000
feet above airport elevation in IMC and 500 feet above airport
elevation in VMC.:
-The
aircraft is on the correct flight path;
Add a penalty of 230 feet per every 10 feet above TCH
-Only small
changes in heading/pitch are necessary to maintain the correct flight path;
If not stabilized the possible effect on
landing distance is Unpredictable
-The
airspeed is not more than VREF + 20kts indicated speed and not less than VREF;
>Vref
add 300 feet per 10 Knots DRY, 500 feet
per 10 knots WET and 2500 feet for each 10 Knots for FLOATING
-The
aircraft is in the correct landing configuration;
If not
stabilized the possible effect on landing distance is Unpredictable
-Sink rate
is no greater than 1000 feet/minute; if an approach requires a sink rate
greater than 1000 feet/minute a special briefing should be conducted;
Corrections
that cause FLOATING can add 230 feet per second
Land where you intend to:
The AFM Actual
Landing Distance accounts for an AIR phase which for the manufacturer has to be
as brief as possible, they usually use methods that we will not be able to
replicate. With that in mind let’s use the normal 3 degree glide slope with a Threshold
Crossing Height of 50 feet. Our air
distance will be at least (50/tan 3) = 954
feet if we follow the glide slope and don’t flare, but we do and usually land
beyond the Aiming Point Markings which begin at 1000 feet down the runway and
end 150 feet later. As you see above,
for every second you fly above the runway beyond the beginning of the Aiming
Point Markings you are consuming precious real estate at a rate of 230 feet per
second. To have not only the markings to
aim for, you can also have the time that takes you from 50 feet over the
threshold to touchdown. Anything above
10 seconds is considered a Long Flare. Try it out, 7 seconds will put you
beyond the Aiming Point Markings and 10 seconds will carry you almost to the
end of the Touch Down Zone which for us is the limit to continue with our
landing. The use of Auto-throttle, HUD,
and Speed increments above Vref which are kept to our touchdown have the
tendency to increase the Landing Distance.
Try it out 7 Mississippi!!!
Do the Math:
CAN YOU STOP? This acronym is a good
beginning.
C–Calculate
Use the manufacturer’s or company
data to determine the landing distance required prior to departure and again
prior to landing based on company SOPs. Use the appropriate factors and be sure
to consider dry/wet runways and associated contamination, planned touchdown
point and speed over the landing threshold, wind speed and direction,
inoperative equipment, and special cases.
U–Understand
The manufacturer’s AFM landing data
is baseline data, and it is derived based on flight test data. Factors should
be applied to the data to adjust it for the current conditions. Pilots should
adhere to the operator’s SOPs and best operating practices which will result in
the safest aeronautical decision making.
S–Stabilize
Ensure that you understand all the
requirements of a stabilized approach and you are able to fly one given the
actual conditions. If not—GO AROUND!
P–Professional
Land like a professional using the
aircraft’s capabilities as described in the AFM and SOPs. A professional puts
safety ahead of style.
Calculate your un-factored landing distance, then
add 30 feet for each knot above Vref dry, add 230 feet for each second of flare
beyond 5 seconds (Add at least two seconds), add any MEL item that affects the
landing distance, add 20 percent to this number for less than maximum braking
and 40 percent for light braking. Down slope will increase the landing distance
10 percent for each degree.
1.
Un-factored AFM landing distance (dry runway)
|
3542
|
2. Airspeed additive
to be held to the landing threshold, e.g. all of the gust. Max additive of 20
knots. Landing distance increase:
Dry
runway: 20-30 feet per knot
Wet
runway: 40-50 feet per knot
Extended
flare: 250 feet per knot
|
250
|
3. Add 2 seconds flare
time due to gusty winds (results in a 230 ft/sec additive)
|
460
|
4. Night–No glide
path–
Assume a 10 foot
error. (Add 200 feet to the landing distance)
|
|
5. Any additions
caused by minimum equipment list (MEL)/ Configuration Deviation List (CDL)
requirements
|
|
6. Subtotal
|
4252
|
7. Runway condition
If wet, add 15 percent
of line 6 or use AFM data if available
|
|
8. Contaminated runway
adjustment to line 6 per AFM and SOPs
|
|
9. Less than maximum
braking add 40% for light breaking and 20% for less than max
|
(40%) 1700
|
10. Total 6+9
|
5952
|
Here you have a
good ball park figure that can be tweaked to accommodate some variables, E.g.
Vref + 10 add another 250 feet, 10
second flare add 690 feet, for a new total of 7268 feet. It can be refined with experience in the
aircraft, the numbers come from ACs and technical publications. Every effort
you make to be precise will pay itself in knowledge and situational awareness.
Go Around:
Many are the reasons
to Go-Around as we read publications and statistics, but apparently we are also
statistically very reluctant to Go-Around and this is an issue that has to do
with each of us in a very personal way. It
is at the end your reason the one that has to be tackled. No matter how many times we have been in an
ALAR course, pushing those levers and saying the words comes from a resolution
within. It comes to that. Why you accept anything other than the
quality you thrive for? Is what has to be addressed.
The correct
choice has to be made well in advance of the flare or the threshold. It is our choice and our decision which has
to be firm even before we see our plane.
Luca Pineda
REFERENCES:
DOT/FAA/AR-07/7
A Study of Normal Operational Landing Performance on Subsonic, Civil,
Narrow-Body Jet Aircraft during Instrument Landing System Approaches
SAFO 06012 Landing Performance Assessment at Time of
Arrival
Stabilized Landings - Charbonneau 2010
USNTPS-FTM-NO. 108 US Naval Test Pilot School FTM
AC No: 91-79 Runway Overrun Prevention
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