Centaurus

Landing over-runs are often caused by excess speed above the published Vref. The B737 FCTM advises increasing Vref by half the steady headwind component and all of the gust when using manual throttle. If the auto-throttle is engaged the recommended approach speed is Vref plus five knots minimum. Boeing also recommend that if a manually flown approach is made the auto-throttle should be switched off.

On occasions the expected gusts or lulls do not occur and as a result excess speed is maintained until touch down with the resultant risks depending on runway conditions.

Where landing length or runway conditions are critical and auto-land not available, then would it be correct to say you can legally leave the autothrottle engaged to 50 feet while flying manually and thus use Vref plus 5 knots regardless of the requirement to add half the HW component and all the gust if conducting a manual throttle approach.

If you have got the aircraft to 50 feet safely without additives (by using the auto-throttle), then this would surely minimise the risk of over-runs associated with excess over the fence speed?

Capt Pit Bull

That's a big if.

AT engaged, AP disengaged, gusty conditions is not a good place to be.

pb

safetypee

Centaurus, your logic has merit. The use of autothrottle also has advantages of reduced workload and reducing any tendency to float. This is particularly so with those auto throttle systems which will retard during manual a flare e.g. Avro RJ. (But it’s not always advantageous to use auto throttle during manual flight).

However, as you realise the overrun problem is not just speed at the threshold. When approaching in gusty conditions, the nature of the gust might result in the aircraft either being high or low at the threshold due to the effect of the gust / lull, this and the same wind variability could also affect the longitudinal position of touchdown.

The risk of an over-run might be reduced if crews thought about the risks in the landing (every landing) when planning the approach. There are many factors affecting the risks, and the more that are known / understood, then with appropriate consideration the overall risk should be minimised.
Where consideration to use auto-throttle is part of the pre-landing briefing, then the crew might also be cued to consider other factors (a trigger for thinking). These too should further minimise the risk of an over-run.

General Refs / Info
1. Managing Threats and Errors During Approach and Landing.
2. AC 91-71 Runway Overrun Prevention.
3. Landing Performance of Large Transport Aeroplane.
4. Running out of runway.
5. Safety aspects of tailwind operations.

electricdeathjet

If I remember correctly the 737 is not an autotrim aircraft..... It should be sporting fun with power going up and down by it self on a manual approach. I think you will be fighting the pitch thrust couple during an approach and the last thing you will be worried about is a couple extra knots over the threshold...

Automatics all ON or Automatics all OFF! I wouldnt mix the two on the tractor

Port Strobe

Don't the Boeing performance figures account for you maintaining Vref+20 on approach and touching down with Vref+gust? Unless the LDA is relatively limiting then I doubt a few knots over the typical Vref additives would be that important so long as there's minimal float associated with the flare, certification does call for average piloting ability to be accounted for. There may be bigger control issues at stake engaging the auto thottle on approach than performance issues without it.

Denti

Yup, they do, well, at least those we use. However Boeing also advises us to bleed all of the headwind additives off over the threshold and only keep the gust additive in, so be prepared to go below the bug (if you bug Vref+ corrections).

Dunno about others, but the lowest normal flap setting for approach on the 737 for us is 30, 15 in case of one engine out and some other non normal conditions, 25 is not an approved landing flap setting (but is allowed for take off). With 30 you need quite a big tailwind to ever go below stabilized criteria N1 (40%) at which point thrust level response is fairly fast. Stabilized means being able to maintain at least that N1 from over 1000ft (we have to be stabilized with landing flaps setting and checklist done by 1000ft AGL) down to flare, if unable a go-around is mandatory.

john_tullamarine

For a typical Vref, one could expect a 20kt additive carried through to result in something in the vicinity of 30-40 percent raw LDR penalty. The scheduled LDR has the 67 percent fudge factor on the pilot's side.

Point is that, on a distance limiting runway, one doesn't want to have too much on ASI ... all a case of horse for courses ... go too slow and one risks falling out out of the sky during the flare (did I hear all the 722 drivers sigh, there ?) .. go too fast and the overrun on a limiting runway becomes an increased probability.

Is there an easy or programmed answer ? Of course not .. due to the varying nature of Mother Nature's winds.

safetypee

Re #7 “… then I doubt a few knots over the typical Vref additives would be that important …”

AC 91-71 “… an aircraft carries 10 knots of excess airspeed through the threshold window on a wet runway surface. The wet runway increase in landing distance would be at least 500 feet, and the extended flare would add another 2,500 feet to the landing distance for a cumulative addition of 3,000 feet.”

These sort of numbers become very important especially if the additives are already greater than 10 kts; but with the assumption “… so long as there's minimal float associated with the flare,” this involves higher workload and appropriate skill to achieve a manoeuvre which is probably not the norm.

For accuracy, the certification requirements for landing state: “The landings may not require exceptional piloting skill or alertness.” (CS 25.125) In addition there is a caveat defining the approach speed:“A stabilised approach, with a calibrated airspeed of not less than Vref, must be maintained down to the 15 m (50 ft) height.”

Any speed additive, altitude deviation, or non standard maneuver, will reduce the margin of safety provided in the landing distances. The margin of safety stems from the factored landing distance.
On a dry runway, the margin is based on measured distance, but on a wet runway, the margin may be significantly less even though the ‘factor’ (physical distance) is larger. In part, this is due to the majority of the distance data used in certification being based on calculations which use a nominal friction. Some manufactures may provide a range of data, i.e. wet or wet/slippery. However, to use this with any confidence crews have to know the runway friction, which is one of the great unknowns in aviation today.
There are also many other variables in the calculation which crews may not have knowledge of, such as runway surface texture and depth of water.
So on a limiting or near limiting wet runway “… a few kts …” might suddenly be very important, particularly if the safety margin is exceeded by the combination of unknowns and excess speed.

xetroV

As far as I know, MD-11 (and I believe also DC-10) SOPs allowed - even encouraged - "manual flight" with auto throttle engaged. On the B737 this is strongly discouraged, except during takeoff and initial climb.

Tony Hirst

The earlier point about trim seems to be the best answer as why it is probably discouraged on approach and encouraged during takeoff and climb out.

I suppose there is an argument for using A/T ARM on approach as it would provide low speed protection.

TidaBisa

I flew for an NG operator in the U.S (700/800) a couple of years ago and we were encouraged to use the A/T even in manual flight. I always wondered about it because the Boeing FCTM does not recommend it. The only time the A/T was not armed was during S/E. Some guys would go manual though. but our SOP was to have the A/T at all times,no winds, gusty winds etc.

Centaurus

Depends how you define the term "spooled up." Regardless of the low drag approach concept, the engines (the 737 series CFM 56) would usually be around 50-55 percent N1 right down to the flare. If the pilot closes the thrust levers on late final because he is too high or too fast, then by definition the aircraft is now not in a stable approach condition and the pilot should go-around. The CFM 56 has different spool up characteristics than the JT8D series engines on the 737-200 and indeed the CFM engines spool up faster. The B737 FCTM does not mention a minimum N1 in relation to an approach to land and this suggests there is no minimum N1 subject to an individual company designated figure. That figure might just be a guesswork number based on someone's personal whim rather than a manufacturer's figure.

Centaurus

Well that's not quite correct, you know. The FCTM wording is: "the gust correction should be maintained to touch-down while the steady headwind correction should be bled off as the airplane approaches touch-down." There is no way you can deliberately bleed off (say) 15 knots - assuming headwind component of 30 knots - between threshold and touch down unless you deliberately prolong the hold off to do so. And that's not on for many well published reasons

The reason behind the HW additive is because of surface friction effect on wind. The windspeed drop off caused by surface friction ceases at around 2000 feet where it becomes known as free stream flow. This means that below 2000 ft (assuming no significant terrain) the pilot should commence the gradual speed reduction in order to arrive at the flare at Vref (plus any gust factor). Of course you would brief the PNF of your intentions in order to avoid the scream of "Speeeed' from the keen and enthusiastic PNF.

See FCTM page 6.11 under sub heading Normal Touchdown Attitude which states: "It shows the airplane attitude at normal touchdown speed for Flaps 30 (Vref30 to Vref 30 minus five knots). With proper airspeed control and thrust management, touch down occurs at no less than Vref minus five.

Providing the correct flare technique is used the aircraft will only lose three knots during the flare manoeuvre. Boeing avoid defining the term "approaching touchdown" with some arguing that short final is approaching touch down after a 1000 mile flight..

mytyresareflat

The FCTM also states that if an automatic approach is being conducted then no wind additive need be included on top of the Vref+5 normal approach speed as the Autothrottle incorporates gust protection...for anyone who has every operated the aircraft in particularly gusty conditions I would be interested to hear their opinion..I prefer to fly manually as the automatics have a lag element when reacting to airspeed fluctuations which in some cases can require very large thrust increases to maintain fly speed and consequently a subsequent manual disconnect can leave certainly leaveyou with extra work to do.