Considerations – How to Flare and Land a Big Jet (v5)

Question

Ricky recently asked me the following question on the QF32 web site:

Good evening Richard,

I read all your books and have been huge inspiration during my training to become a pilot. I read you have flown nearly all airplanes and in the book you also mention a technique used on Caribou airplane.

Now I’m flying PA28 and a problem I’ve had during my training is to start flaring quite high, basically I look to the aiming point (abeam the PAPI) then when over the threshold I start looking to the far end of the runway, and Immediately my perception is to start gradually pulling as I feel a high closure rate. The result is flaring high and running out of energy too high.

My instructor doesn’t give much help in this as the common answer is “you’ll develop with experience “. In your opinion how can I improve? Maybe before i posted on the wrong section of this website 

 

FLY! – the Elements of Resilience

Here is my Answer (for Big Jets)

Hi Riky,

Flying a stable approach to landing is one of the most difficult manoeuvres pilots do in commercial aviation and poor-unstable approaches are almost inevitably contributing factors in most landing accidents. So it’s good to get a few basics correct about how to consistently make an safe approach and landing.

Please note: The following is written to suit Big (commercial airline) jets.   Whilst your small aircraft is certified to standards that are different to the (Part 25) standards for the Big Jets, many of the concepts remain relevant for smaller types.

Remember, practice builds habits.

The keys to a good approach and landing in big jets are:

  1. Know your runway length (performance) and width (flare height perception)
  2. Avoid tailwinds if possible
  3. Fly the correct approach-glidepath angle
  4. Approach at the correct airspeed 1.3Vs / 1.23 Vs1g
  5. Flare at the correct height
  6. Focus on the aim-point to the flare height
  7. At the flare height lift your eyes to focus at the end of the runway
  8. Touchdown in the correct area
  9. Fly the aircraft onto the ground
  10. Correct rate of Descent at touchdown
  11. Don’t float
  12. Summary

1. Know your runway length, width and aircraft performance.

Know your aircraft performance and the excess runways length for your landing.  (Variables include:  runway surface condition & length & elevation, obstacles, wind, temperature, atmospheric pressure, aircraft configuration & condition & weight & approach speed.)

Know the runway width.  Pilots tend to flare high and float on runways that are wider than the runways they are (habitually) accustomed to. This happens because the expanding field of view of the runway (in the proportion to our total peripheral vision) happens earlier on wider runways than it does for narrower runways.

2. Avoid tailwinds if possible

For a given (indicted) approach speed (IAS), tailwinds increase your ground speed (GS), and your rate of descent (ROD) (when flying a defined approach path):

  • reducing approach thrust – increasing time to spool up the engines in a go-around – increasing height lost below the decision heights during aborted approaches
  • increasing the risk for overstressing the landing gear
  • increasing aircraft energy and stopping distance

GS = IAS + Wind effect

3. Fly the correct approach-glidepath angle

It’s important to fly the correct approach path and airspeed because when you do this, you will also maintain the correct rate of descent as you approach the airfield to flare and land. If you come in to steep then your rate of descent is higher and you need to flare earlier-faster to arrest the rate of descent before touchdown.

The normal approach angle is 3 degrees or 5.24%. This angle provides clearance from the Obstacle Free Zone (OFZ) that is carved by airport designers for your aircraft type and type of approach. The OFZ provides clearance from trees, obstacles and terrain.

For fast jets, the three degree glidepath also enables the desired rate of descent (ROD) to be easily calculated (using your GPS) as approximately “five times your groundspeed (GS) + 50″ feet per minute.    For example, if you are approaching at:

  • 180 kts GS, then your ROD should be 900 + 50 = 950 feet/min.   (within 0.4% of actual 954 fpm)
  • 160 kts GS, then your ROD should be 800 + 50 = 850 feet/min.   (within 0.2% of actual 848 fpm)
  • 1400 kts GS, then your ROD should be 700 + 50 = 750 feet/min.   (within 1.1% of actual 742 fpm)
  • 120 kts GS, then your ROD should be 600 + 50 = 650 feet/min.   (within 2.2% of actual 636 fpm))

For light aircraft  a similar simple rule (for a three degree glidepath) is the desired rate of descent (ROD) is approximately “five times your groundspeed (GS) + 25″ feet per minute.  For example, If you are approaching at:

  • 120 kts GS, then your ROD should be 600 + 25 = 625 feet/min.   (within 1.7% of actual 636 fpm)
  • 100 kts GS, then your ROD should be 500 + 25 = 525 feet/min.   (within 0.9% of actual 530 fpm))
  • 80 kts GS, then your ROD should be 400 + 25 = 425 feet/min.   (within 0.2% of actual 419 fpm))
  • 60 kts GS, then your ROD should be 300 + 25 = 325 feet/min.   (within 2.2% of actual 318 fpm))

When you do this calculation often enough, you can do it quickly in your head using a GS (NOT IAS) readout.   You can check and confirm your actual glidepath by making this calculation (using indicated GS and ROD readings) any time during the approach.

Another way to calculate “5 x” is to halve the number and add a zero.

Do not underrate this procedure.   I have used this calculation my entire career to confirm my approach path during every approach – even for the last tens years while flying the A380.   (An incorrect ROD during an ILS approach may indicate you are flying a false glideslope (sidelobe) signal.)

The PAPI / VASI should be aligned to your approach path.

Certification Limitations:

  1. Large aircraft are certified to sustain maximum RODs of 600 FPM (10 Feet Per Second (FPS) at touchdown.
  2. Landing Gear shock absorbers are designed to take a ROD of 720 FPM / 12 FPS at touchdown.

4. Approach at the correct airspeed (IAS) 1.3Vs / 1.23 Vs1g

In 1943 the FAA designed landing performance with the aircraft at 50′ over the end of the runway at a speed equal to its stall speed + 30% (1.3 VS).   If you fly slower than this speed, then you risk not having sufficient lift to flare and arrest the rate of descent during the flare and continued airspeed bleed below 50 feet.

Because manufacturers test pilots “fudged” their flying techniques when calculating the VS speeds, the FAA in the late 1980s changed the 1.3Vs speed to a more practicable and “unfudgeable” 1.23 VS1g. (VS1g is the speed where the lift equals the aircraft’s mass. It’s also called the “G Break” speed).

All aircraft certified after the mid 1990s are certified using Vs1g.

Be aware that tailwinds add significant risks to safe landings. For a constant IAS, not only does your groundspeed (GS) increase with tailwinds, but your kinetic energy increases with the square of the GS.

Garuda Indonesia Flight 200‘s approach to Yogyakarta was at a classic example of excess energy.   The 737 touched down 860m beyond the runway threshold at a speed of 221 knots (409 km/h; 254 mph), 87 knots (161 km/h; 100 mph) faster than the normal landing speed.  That aircraft had an energy on touchdown 221^2/134^2 = 2.7 times the normal energy on a normal approach.

5. Get the correct Flare Height

It’s important to commence your flare at the correct height and develop a consistent flare to touchdown.

The ideal flare height depends upon many factors, but in ideal conditions (nil wind, no runway slope, corect IAS, 3 degrees approach angle), you will commence the flare at a constant height.

If your flight manual has no information or you are unsure about calculating this height then the Jacobson Flare website and software might be of help.

The newer big jets provide aural alerts from the radio altimeter calibrated to measure the height of the wheels above the ground.

For example, the A380 has RAD ALT aural callouts from at least “ONE HUNDRED” feet down in ten foot decrements to “TEN” and finally “FIVE” feet.  These callouts provide another sense not just of the wheel height, but by their separation in time, the rate of descent.

The official A380 flare height is around 40 feet.  Experienced pilots subconsciously incorporate the RAD ALT callouts into their body model (that includes the aircraft) so well that it’s possible to sense (or “feel”) the wheel heights (that are about 29  metres behind and 10 metres below the pilots’ eye heights) to within (I think) +/- 15 cm.  This is why experienced pilots can land the A380 so smoothly.

 

(Photo RDC)

6. Focus on the aimpoint to the flare height

It’s important to keep your eyes on your aimpoint all the way through to reaching your flare height. When you focus on the correct aim-point, then you are guaranteeing setting up the aircraft to reach 50 feet at the runway threshold, and thus also guaranteeing your aircraft performance.

DO NOT lift your eyes up to look at the end of the runway as you approach the flare height. If you do, then you will unconsciously pull back on the controls, causing the aircraft to pitch up, float and land long.

As you approach the runway focussing on the aim point, you will sense the runway (and the ground environment) expand (widen in angle) in your peripheral vision.   For a constant angle to the sides of the runway in your peripheral vision, wide runways will reach this angle at a higher altitude than narrower runways.  It’s because of this altered peripheral perspectives that all (even experienced) pilots tend to (incorrectly) flare high and float on wider runways.

It’s for this reason that you MUST know (and adjust your flare height for) the width of your runway.

When your mind subconsciously matches its sensory inputs with the pattern of senses remembered at flare heights from previous ideal landings, then your mind subconsciously triggers a habitual response in your cerebellum to move the muscles required to commence the flare.

The only ways to build the sensory memory of the correct flare point is to either follow the practices of the instructors that consistently get the flare right, or study the Jacobson Flare above. Remember, practice builds habits.

7.  At the flare height lift your eyes to focus at the end of the runway.

When your peripheral vision provides the sensory input that you have reached your flare height, then this is the time when you should look up and focus your eyes towards the end of the runway and commence your flare.

When you look up, you now concentrate on the touchdown. Your peripheral sight provides your sense of your rate of descent, helping you arrest it and land at the correct rate of descent.

8.  Touchdown in the correct area

Vale Eric Melrose Winkle Brown (1919-2016).  One of the World’s best test pilots.

The role of the approach is to position you at the runway threshold at 50 feet at a steady speed of 1.3VS / 1.23 VS1g. If you achieved this, then your approach was perfect. Now it’s time to land…

Landing performance is calculated using an Air Segment (commencing 50 feet over the runway threshold at the correct approach (IAS) speed, and a Ground roll Segment. The ideal air segment is about 7 seconds from 50 feet to touchdown.

The design rule of 7 seconds from 50′ to touchdown should be your guide. Any time quicker indicates an increased risk of a shorter landing and harder touchdown.  Anything longer than 7 seconds should trigger a flag that you are probably operating outside the design criteria for your landing.

9.  Fly the aircraft onto the ground

At the flare height:

  • Lift your eyes to focus on the end of the runway.
  • Pitch the aircraft nose-up slightly to achieve a gradual reduction in the rate of descent (that your peripheral vision will sense).
  • When your rate of descent reduces to the rate of descent you wish to land with, then hold that attitude (by looking at the angle of the dashboard/cowling to the horizon)  until the aircraft settles on the ground.

Adarsh, Texas, USA (Photo: Vidhya)

10.  Correct Touchdown Rate of Descent

During the touchdown keep you eyes at the end of the runway as you fly your aircraft to the ground.

At touchdown, the designs have you slowed to 5 knots below 1.3VS / 1.23VS1G (lose 5 kts in the flare) and so you still have heaps of excess speed to lose.  So if you pull the nose too high, then you will float!

The harshness of the intended touchdown should vary with conditions.

  • If the runway is wet or slippery, then a harder touchdown is warranted to have the tyre push through and displace the viscous layers to the runway surface.
  • If there is a strong crosswind then it’s good to get the wheels firmly on the ground quickly.
  • If it’s a dry runway, then you can afford to finesse the touchdown.

Here’s guidance for the egotists who strive for a  smooth landing every time.  A smooth landing is only impressive or even acceptable when:

  • you touch down within 7 seconds after passing the threshold at 50 feet.  A smooth touchdown in any other situation could be compromising performance margins and therefore increasing your risks of a float/overrun,  and
  • landing on a dry runway, or
  • landing on an uncontaminated or wet runway at a speed that is below your aquaplaning speed. (Vaquaplane kts = 9 times the square root (tyre pressure psi)

11.  Don’t Float

Most people don’t realise why floating down the runway is dangerous. So here is the reason you should NOT float.

The Coefficient of Lift Induced Drag (Cdi) reduces exponentially as the aircraft approaches below the height that is a few of its wing spans above the ground.

The Prandtl’s Lifting Line Theory states that CDi (induced drag) reduces to ~ 50% at 10% of the wingspan ground clearance.  This reduction in Cdi is called “Ground Effect”. When the drag reduces, your deceleration reduces. So an aircraft flying in ground effect will decelerate at a slower rate than an aircraft outside ground effect.

When arriving at 50 feet the correct speed of 1.3VS/1.23 VS1g, your aircraft has excess speed (above the stall speed) that serves as a safety margin during the flare, deceleration and landing.

The aircraft In Ground Effect (IGE) has significantly lower drag than an aircraft Out of Ground Effect (OGE).   Helicopter pilots know so much about this phenomenon that they have separate IGE and OGE performance charts.

If you try skimming along the runway IGE to hunt for that smooth landing and hope the airspeed will reduce quickly as you descend to the ground, then you are in for a shock.  A big jet IGE  takes a hopelessly long time to slow appreciably.   If you hold the aircraft just off the runway IGE to perfect a landing, then the airspeed will not reduce quickly and you will quickly find yourself with too little remaining runway to land and stop safely.

Here is Weiselberger’s theory (graph) for a 747-400 (213’ wingspan), increased by 12’ for the winglets. The graph shows the ground effect during an approach and Induced Drag.   The graph shows CDi is down to just 20% at 10 feet at later parts of the flare!

InducedDrag_Vs_Alt

The reduction in CDi is why:

  • Do not allow the airplane to float, fly the airplane (well above its stall speed) onto the runway.
  • Do not attempt to extend the flare by increasing pitch attitude in an attempt to achieve a perfectly smooth touchdown.
  • After main wheel touchdown, do not attempt to hold the nose wheel off the runway to slow the aircraft, because:
    • you reduce the force on the braked wheels and their braking efficiency
    • logic to fully deploy spoilers, speedbrakes and brakes might be delayed

12.  Summary (for Big Jets)

  • Good landings only ever follow equally good approaches to 50 feet.

  • Practice builds habits.   Practice your landings until the flare height and flare procedure becomes habitual.
  • Know when to transition your focus from the aim point to the runway end.
  • Fly the aircraft onto the ground – don’t float.
  • Know your aircraft’s aquaplaning speed.
  • A smooth landing is only impressive when it’s within certification limits and safe.

 

Version 4    Additional ROD figures provided as a guide (19 Dec 2019)
Version 3    14 Dec 2019
Version 2.   Incorrect energy increase in section 4 corrected to be 2.7.  (thanks Gordon)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: