What is the vertical speed (range) of a Boeing 787 (or any other large commercial aircraft) that should be achieved just before and during touchdown? I'm not interested in the vertical speed during the approach, my question only targets the last 100 feet before touchdown... this includes the vertical speed after the flare. Obviously a vertical speed of 0 fpm is desired for comfort reasons, but as I understand (see also this question), aiming for 0 fpm makes it difficult for pilots to aim at an optimal touchdown point on the runway.
From an MIT ICAT paper:
Today's commercial autoland systems utilize the ILS (...) At around 30 ft above the ground, the aircraft enters the flare mode, during which the aircraft pitches up to reduce the vertical speed from its current descent rate to 1-3 ft/s by touchdown.
Which means we're looking at 60-180 FPM touchdown rate.
Kiss landings (near 0 FPM) are undesirable, because they can't be achieved on regular basis without risking long landings. They can also break landing gears (more on that below).
I checked various Boeing/Airbus operating manuals, they do not state the target rate of the flare mode, but from the 747-400 flight crew training manual (page 6.11):
- Do not allow the airplane to float: fly the airplane onto the runway.
- Do not extend the flare by increasing pitch attitude in an attempt to achieve a perfectly smooth touchdown.
Landing with extremely low sink rates is more likely to experience shimmy than a firmer landing because the torsion links remain in an extended vertical position, where the damper has less mechanical advantage for longer periods of time (Boeing).
Boeing commercial aircraft are designed for 600 FPM landings, with 360 FPM when above the maximum landing weight. Boeing also says pilots are the best evaluators of hard landings, "because of the difficulty in interpreting recorded acceleration values at the CG of the airplane."
Based on pilot reports and data, exceeding 240 FPM is a hard landing, which further confirms the 60-180 FPM to be an ideal firm value whether the landing is flown manually, or with the auto-land (Boeing).
The aircraft should descend on a straight line, typically 3 degrees, up to the flare. This corresponds to a vertical speed of 600 feet per minute if landing airspeed is 120 knots; higher is possible. During the flare, nose angle increases but throttle decreases, so the effect on sink rate will depend on several factors. The following manual mentions 150 feet per minute as a typical touchdown speed.
A vertical speed of 0 is not feasible nor desirable, because descending the last few feet will take "forever." For example at VS = 20 fpm, descending the last 10 feet will take .5 minutes during which the aircraft will fly forward about 1 mile!
From 737 NG Flight Crew Training Manual, 737-600 Touchdown Body >Attitudes, Section 6, 2008. Typical landing conditions:
• 3° approach glide path
• flare distance is approximately 1,000 to 2,000 feet beyond the threshold
• typical landing flare times range from 4 to 8 seconds and are a function of approach speed
• airplane body attitudes are based upon typical landing weights, flaps 30, VREF 30 + 5 (approach) and VREF 30 + 0 (landing)
A smooth thrust reduction to idle also assists in controlling the natural nose down pitch change associated with thrust reduction.
The closer to zero the better! Obviously there has to be something or you'll stay flying forever (run out of runway and have to go around) but the most comfortable for the plane and passengers is to touchdown at near zero vertical speed during the last few seconds of the flare.
EDIT: Based on user mins observation of you stating not wanting the VS on approach you can ignore the last paragraph but essentially during the flare you are going from approach VS of about 750 to a final touchdown of as close to 0fpm as possible.
As for the approach portion before the flare it can be figured out with some math based on the ground speed and the angle of the approach (typically 3 degrees on an ILS approach). I'm not going to go deep into the vector equations but there are some nice Rules of Thumb you can use to figure it out instead. For example multiplying the groundspeed* by 5. I think a 787 does about 150KIAS (Knots Indicated Airspeed*) on approach typically so that would work out to 750fpm.
[*] Of course depending on the headwind or lack thereof it will change the groundspeed and thus the rate needed to maintain the 3 degree (or whatever it is) on approach.