It is not uncommon top be slightly low on the glideslope when crossing the threshold. Used to do it intentionally on the occasional short runway where we were in tight confines in the old days before the aircraft were so closely monitored. On the turboprops, we really used to "dip the PAPI's" close in. I remember one particularly short runway where the PAPIs had you landing halfway down the runway. Unsafe. Still did it in the narrowbody jets at one location that was max landing weight and runway limited weight together. Haven't felt the need to on the widebodies....yet.

Is there really any difference between a smaller jet and a larger jet in the threshold crossing height? As far as I know there is an adjustment in the avionics in order to ensure a similar gear clearance over the threshold.

For example, the Boeing FCTM's have a table for "main gear over threshold" with pilot eye height over threshold/main gear height over the threshold. For the 737-600 the two numbers for a 3.0 degree glide path are 49'/33'. For the 747-400 the numbers are 66'/31'. So being a dot low on either is practically the same reduction in main gear clearance. The real difference between the widebody and the narrowbody is the pilot sitting higher for a given glideslope location.

Who hasn't seen near full scale glideslope deflecton below 100 feet? You might need that extra couple of hundred feet of rollout on a slippery runway that was supposedly "braking action reported as fair/medium".

You might need that extra couple of hundred feet of rollout on a slippery runway that was supposedly "braking action reported as fair/medium".

Especially in the non-normal landing direction, on a rainy day, with all the rubber at the stopping end.
I used to adjust flaps & thrust for takeoff in a similar situation; in case of an RTO to gain a few more meters. It caused various interesting reactions from the other seat when we 'didn't just use the numbers'. But that's another topic and I do to wish to 'creep the thread'.

Years ago a lot of these personal techniques were common in my experience. Folks would add knots to the approach speed for the wife and kids. Non-standard additives would be made for wind with autothrottles in use.

Seems like some of the Air Force guys would brief 'I'm going to duck under a dot on short final since the viz is good' while landing a widebody. Was it taught in the T-38, the C-141 or the C-5 perhaps?

One of the regional carriers decades ago had a non-approved B-737 short field technique of pulling up the speedbrake handle before touchdown so that the boards would fully deploy the moment there was weight on the wheels. When a newer version of the 737 arrived, ground spoiler mode was triggered by radar altitude, the boards fully deployed before the wheels were on the runway and a hard landing mishap occurred.

In recent years, thankfully, most of these cowboy techniques seem to be less common where I've worked. However, with all the airline 'mergers' I'm sure non-standard 'we did it this way at Brand X' procedures will give the feds and the training department a lot of job security for years to come.

Doors to Automatic asked if there is a set threshold crossing height or if discretion can be used by the PF.

There is a published threshold crossing height yes. Judging from the replies there's no consensus about discretion of going low. Could it be the ones who fly into short/slippery runways in small(er) jets and turboprops take it lighter than those flying widebody jets?

Crossing the threshold how many feet lower does being a half dot low equate to? Or a full dot?

"near full scale glideslope deflection below 100 feet"

"how many feet lower does being a half dot low equate to? Or a full dot"

Depending on the type and location of GS ground installation the aircraft indications will progressively show below the ideal path because of the parabolic nature of a reflected beam. Thus the GS scale is not a suitable indication of height or deviation at lower altitudes.

Crossing the threshold a half dot, or a dot, is a relatively small displacement. IMO a bigger issue is demanding "don't do this, don't do this" so guys make relatively large changes in aimpoint/touchdown point because of an over concern about being slightly low.

At 100' a half dot is roughly 2.5 feet. A dot is five feet(+/-). If your aimpoint/flight path is correct you'll cross the threshold 1-2' from ideal. That's not the end of the world. Making a significant correction and losing your aimpoint/flight path, and perhaps requiring a secondary correction, puts you at risk of a bigger deviation from acceptable performance.

Hmm. Rule of thumb for stabised approaches (1 dot high max, no lower than 0,5 below) is the error becomes +20 % and -10% of the height. Your numbers would be wrong by factor of four. Anybody care to do the math?

By the way I'm in the "aim to land as calculated using as much muscle memory/visual drill as possible". The humble observation being, that "out of the box" / "this one time only" techiques make for longer landings, at least on 50% of the attempts. Uglier ones for about 90%. (A320)

As I said, we used to land turboprops(no reverse by the way on the engines) on a short runway less than 3000 feet(and some that were a bit longer). Was 100% hard pack snow all winter.

Now some folks might call you a cowboy if you don't follow the official flight path all the way down and land at about 13-1500' down the runway(which has a downslope in both directions for the latter half of the rollout) because you followed the PAPI and had your 50' wheel clearance....

...but in reality, you would be a fool if you were anything but a so-called cowboy. And yes, it was done during a check flight. Some 10,000 foot plus runway guys have a hard time understanding stuff like this.

Why the gripe? Different OPS, different standard techniques... Carry one across to the other field, and you're set for trouble.

BTW it is not wheels at 50', but G/S antenna, the wheels are at 33 over the greens. LOT of difference. 33' wheel clearance also illustrates, why "going low" brings little benefit.

Airbus: pilot eye / antenna / gear
320: 56 / 50 / 34
330: 59 / 50 / 28

There seems to be an excessive amount of 'this is what I did' versus 'this is what they did'.

How do you recognize an Airline Pilot at a party, he'll tell you comes to mind.
Enough about me let's talk about my boat.

What you did 30-40 years ago in your 3-holers has no relevance to this accident.
Why they were doing 130kts GS 4600' down the runway does.

FlightDetent, re calculating deviation in feet from dots above/below the glideslope and, "Anybody care to do the math?"

I looked this up some time ago on PPRuNe when I was working on FDM Program determination of touchdown points to ensure the correctly-calculated distance from 50' to touchdown.

The matter was discussed, along with the mathematics, at:

http://www.pprune.org/questions/3622...ml#post4719685

That post suggest's one dot below equals to 0,2 degress angular displacement. Some other sources on the web state 0,15 and 0,35. Difference too large to draw conclusions. Off the topic, anyhow.

Thanks still.

There isn't a "standard dot". Some instruments have 5 dots per side (full deflection), while others might have only 2 dots or 3 dots for the same deflection. At the extremes I've seen older instruments with no dots at all and one with 10 dots per side.

Assuming a 3 degree GP, on a 5-dot display one dot is approx. 0.15 degrees. On a 2-dot display, one dot is approx. 0.35 degrees.

Has anyone closed in on factual truth behind this over-run. One assumes the landing performance was adequate, so what lined up a few holes? All this talk of dots & degrees & slopes & displacements is giving me a headache. KISS. Keep the crash point fixed in the window at the correct speed and you should have a modicum of success more often than not.

B2N2, re, "Why they were doing 130kts GS 4600' down the runway . . . ", where'd you get the "130kts" & "4600ft" from?

Firstly, with as little personal input as possible:

https://aviation-safety.net/wikibase/wiki.php?id=190959

https://www.flightradar24.com/data/a...n278ea#b70e06f

https://s19.postimg.org/nj5b30wq7/pence_chart.png

https://s19.postimg.org/674yhl38v/pence_raw.png

my notes:
. FR24 claim to use "FAA data", what it means I do not know, however the recorded altitude seems to be corrected for QNH (which is not the case with ADS-B raw values)
- For ADS-B raw data, speed = ground speed
- Typically for ADS-B raw data, altitude = 0 means "positive weight on wheels", not the actual value
- ALT = 0 at time 23:40:34 should be interpreted with extreme caution as the speed for that frame is invalid.

Based on the above, the time lapse to slow down on ground from 135 >> 132 kt is 18 seconds, which translates to 1200 meters of distance covered. This is ignoring the suspicious reading at 23:40:34.

Fact: LDA = 2100 m
Observation: B2N2's quoted figure of 4600 ft = 1400 m.

Link provided by PJ2 shows 1 dot = 6.6' at 100' AGL.

So one dot = roughly 3' crossing the threshold.
1/2 dot = 2'(rounding error).

I'm not saying ignore the vertical deviation but IMO aim point is a significant factor in the entire process.

FlightDetent;

Oh, okay, thank you. I just took it for granted that with a number that precise, that accuracy was also implied from an NTSB communication or something that I'd missed.

That kind of accuracy can't be done from FR24 etc., for the reasons you've stated so we'll see what the Interim Report says.

Other than ADS-B, sites like FR24 get a "near real-time" data feed from the FAA (called the ASD feed). This feed contains flight plan and position data (including lat / long, ground speed) for all IFR aircraft in the US air space.