Final approach alignment

28.10.2004

Submission:

Pans-Ops Paragraph 6.2 on page 3-33 mentions that the MINIMUM distance from TH to final approach course/extended centreline intersection is 1400m. The picture on the next page also shows 1400m as the minimum distance. However later in the paragraph under the Minimum OCH chart it says that the values in the chart take into account the MAXIMUM interception distance of 1400m. I know TERPS pretty well so I am wondering if that 1400m is the MAXIMUM distance which would match up with the TERPS maximum distance of 5200''. I just wanted to know if the final course can intercept the extended runway centreline anywhere between TH and a maximum distance of 1400m or 1400m and out.

Bill Frenz

The MITRE Corporation - Center for Advanced Aviation System Development

Answer or Commentary:

(I.W.

The short answer is that the final course must intercept the extended centreline before the 1400m point if not within 5° of the extended centreline

I think there is a misprint in the manual, after the table on that page the word "maximum" should be "minimum". Before amendment 10 the gate used to be 150m wide at a distance of 900m from the threshold and no aircraft category was specified. I assume that because of the difficulty that some of the larger aircraft had turning onto short final the distance of 900m was increased to 1400 m and the varying intercept angles for different aircraft categories added.

While we are on the point I think the formula on that page may need to be explained for anybody to use it.

Minimum OCH = 15m + Lxp%

With L = 1400 m + r tan (q /2) + 5 s of flight


Lxp% is the descent gradient as a decimal multiplied by the calculated value of L. If the descent gradient is 5% then

Lxp% => L * 0.05


"r tan (q /2)" this is the radius of the turn multiplied by the tangent of half the amount of offset from the runway centreline. The value r can be calculated from the formula in the green pages (III-E-1). Remember the bank angle is 15° (not the same as q ) and the speed (V) is the True Air Speed (TAS) of the aircraft category. The TAS can be calculated by using the maximum Vat value in the tables on page 3-5, combining it with the altitude of the aerodrome, ISA +15 and the tables on green pages III-F-2/3. You can also use this TAS value to calculate the 5 s of flight. However remember that your IAS speed is probably in Knots and the result will be in Nautical miles not meters as is required by this formula.

If you sit down and do the maths you end up with OCH values that are very close to the ones in the table but not exactly. Does anyone know the actual altitude and ISA values that were used in the table determination?)