Threshold height

[ElNull]

hi

"Special attention must be given to establishing a final approach that assures safe threshold clearance and gear touchdown at least 1,000 feet down the runway."

what is the purpose of the Threshold height or clearance? why would we take a clearance from the threshold?

(sorry this might be a stupid question to some of you)

[411A]

Because, with some (most) heavy jets aircraft, the main landing gear is well behind, and lower, than the pilots eye level.
With the L1011, as just one example, the MLG is 79 feet behind, and 38 feet lower than the pilot (in the landing configuration attitude) so....IF you get too low, sufficient approach light clearance is not guaranteed.
Therefore, there is a minimum threshold crossing height (TCH) established...in the case of the L1011, it is 42 feet.
The TCH is normally printed on the specific approach chart.

[ElNull]

thanks for your reply 411A..

If I get too low (no sufficient clearance from MLG and threshold) why would this be a problem? am I not allowed to land at the Threshold?

[galaxy flyer]

Try it often enough and you WILL land short of the threshold! TCH is an important parameter, consistently crossing the threshold at somewhere between 40 and 60 feet is necessary.

GF

[TopBunk]

I seem to recall that CCS (Caracas) is quite interesting!

Landing there some years ago in a B747-400 on what was then runway 09, but now seems to be 10, I was in the P3 position on my first visit when I witnessed the rad alt call out '100' followed instantly by '30', by which tie it was almost too late to flare, and a positive arrival followed.

This was caused by a ravine running up to almost the threshold, and the TCH was quite low anyway.

I stored this away and when landing there a couple of days later I must have been about half a dot high on the glide. The RA called 100 so I flared, only for it to call '50' about a second later, by which time I was at Vref and decreasing! We landed ok at about Vref-5 or so, but I was grateful for the mass and momentum of the B747 that saved the day!

[NightDog83]

again Hg is important and obstacles too. In the example cited, if you have a TCH of 50 feet and the gear is significnatly lower like 20 feet, you are really not clearing ther threshold by that much. Add a small obstacle in close and trouble. I think for low visibility approaches in US, it the TCH is less than 50 feet, the operator needs to perform an obstacle assessment. Nightdog83, sir.

[s_irus]

Hi all

Understand the standard vasis at 3 degree will bring us to 15m eye level above threshold, can anybody tell me where I can find official writeup on 2-bar vasi? I tried looking up in ICAO website but to no avail. Any help would be very much appreciated thanks.

[STOLskunkworks]

This is from the Canadian AIM and should be ICAO standard but I can not vouch for the accuracy of this info outside Canada.

7.6 App roach Slope Indicator Systems
7.6.1 General
An approach slope indicator consists of a series of lights visible
from at least 4 NM (2.5 NM for abbreviated installations)
designed to provide visual indications of the desired approach
slope to a runway (usually 3˚). Aircraft following the on-slope
signal are provided with safe obstruction clearance within 6˚
to 9˚ on either side of the extended centreline out to 4 NM
from the runway threshold. Exceptions will be noted in the
CFS. Descent using an approach slope indicator should not be
initiated until the aircraft is visually aligned with the runway.
Approach slope indicator systems provide safe wheel clearance
over the runway threshold. The vertical distance from a pilot’s
eyes to the lowest portion of the aircraft in the landing attitude
is called the EWH, and this distance varies from less than
four feet to up to 45 ft for some wide-bodied aircraft, such
as the B-747. Consequently, approach slope indicator systems
are related to the EWH for the aircraft that the aerodrome is
intended to serve and provide safe wheel clearance over the
threshold when the pilot is receiving the on-slope indication.
The Canadian civil standard for VASIS and PAPI has the
lights normally situated on the left side of the runway only.
When available strip widths preclude the use of a full system,
an abbreviated approach slope indicator consisting of only two
light units may be installed.
Approach slope indicator systems are categorized as follows:

Visual Approach Slope Indicator System (VASIS)
V1: 2-BAR VASIS intended to serve aircraft with an
EWH up to three metres (ten feet).
V2: 2-BAR VASIS intended to serve aircraft with an
EWH up to 7.5 m (25 ft).
V3: 3-BAR VASIS intended to serve wide-bodied aircraft
with an EWH up to 14 m (45 ft).
AV: AVASIS intended to serve aircraft with EWH up to
three metres (ten feet).
PAPI (Precision Approach Path Indicator)
P1: PAPI for aircraft with an EWH up to three metres
(ten feet).
P2: PAPI for aircraft with an EWH up to 7.5 m (25 ft).
P3: PAPI for aircraft with an EWH up to 14 m (45 ft).
AP: APAPI for aircraft with an EWH up to three metres
(ten feet).

7.6.2 2-BAR VASIS (V1 and V2)
The 2-BAR VASIS (V1 and V2) consists of four light units
situated on the left side of the runway in the form of two pairs
of wing bars referred to as the upwind and downwind wing
bars. The wing bars project a beam of light having a white
colour in the upper part and a red colour in the lower part.
(a) When you are on the approach slope, the upwind bar will
show red and the downwind bar will show white.
(b) When you are above the approach slope, both upwind and
downwind bars will show white.
(c) When you are below the approach slope, both upwind and
downwind bars will show red.
(d) When you are well below the approach slope, the lights of
the two wing bars will merge into one red signal.

7.6.3 3-BAR VASIS (V3)
The 3-BAR VASIS (V3) is basically a 2-BAR VASIS (V2)
with one light unit added to form an additional upwind bar.
This provides a greater threshold wheel clearance for aircraft
with a large EWH (a wide body). The system then consists of
three wing bars as follows:
• upwind bar (added)
• middle bar (upwind bar of V2)
• downwind bar of V2
Wide-bodied aircraft use the upwind and middle bars to
provide safe wheel clearance and conventional aircraft (up
to 7.5 m (25 ft) EWH) use the middle and downwind bars as
with V2.

Where VASIS is provided on a precision approach runway, it
will be turned off in weather conditions of less than 500 ft
ceiling and/or visibility less than one mile, unless specifically
requested by the pilot. This is to avoid possible contradiction
between the precision approach and VASIS glide paths.

7.6.4 Precision Approach Path Indicator (PAPI)
PAPI consists of four light units situated on the left side of the
runway in the form of a wing bar.
(a) When you are on the approach slope, the two units nearest
the runway show red and the two units furthest from the
runway show white.
(b) When you are slightly above the approach slope, the one
unit nearest the runway shows red and the other three show
white. When you are further above the approach slope, all
four units show white.
(c) When you are slightly below the approach slope, the three
units nearest the runway show red and the other white.
When you are well below the approach slope, all four units
show red.
(d) Although the single wing bar configuration remains the
same for all PAPI systems, it is possible to provide for
safe wheel clearance over the threshold for aircraft with
different EWHs, i.e., P1, P2 and P3 for aircraft with an
EWH of up to 3 m (10 ft), 7.5 m (25 ft) and 14 m (45 ft)
respectively, by varying the distance of the wing bar from
the runway threshold.