RNAV flown till -15°Celsius but NDB not

[Speedwinner]

Hello folks,

please can somebody explain me in simple words why the RNAV-APP can be used just till -15°C and the ndb and other NPApproaches have no limit?

And when i fly an ils approach at -20°C OAT, do i intercept the glide later or earlier? It doenst really matter because the glide is fix and not temperature related is that right?

Sorry, my mind is blocked;-)

Greets

[skyhigher]

Altitude is temperature related, in that the altimeter is calibrated at ISA. So colder than ISA is a lower altitude. Thats why cold weather correction tables are included in ops manuals. From memory a rough way of correcting is 4% per 10 degree devaition from ISA, but i use the table in the ops manual.

Why the restriction on RNAV approaches i dont know.

[engfireleft]

The RNAV approach you refer to is probably LNAV/VNAV which must be flown vertically guided, but relies on BARO-VNAV to calculate the vertical path. The approach is designed like an ILS in that the obstacle clearance is a sloping path down to the runway. But because the path is based on barometrics, in cold temperatures that path is lower to the ground unlike an ILS glideslope that remains constant regardless of temperature. When designers build the approach they calculate the lowest temperature that the required obstacle clearance is still sufficient. That temperature limit (TLIM) appears on the chart, and below that temperature the LNAV/VNAV approach cannot be flown. Note: MDA must still be corrected for temperature even though FAF crossing altitude is not.

Conventional non-precision approaches do not have a temperature limit because you are supposed to correct the FAF and MDA altitudes during cold weather. This ensures the required obstacle clearance is maintained.

You may also see LPV limits on an RNAV approach chart. They too must be flown vertically guided, but unlike the LNAV/VNAV approach it is not flown using BARO-VNAV. They are flown from point to point in space in 3 dimensions (including height) from position information derived from spacial navigation. It has no barometric input, and therefore is not effected by temperature.

You are correct about ILS glideslopes not being effected by temperature because it is an actual radio beam projected from the ground. What does change however is the indicated altitude at which you will cross the FAF, even though your actual height above ground remains the same.

[ant1]

Would it be too much asking to have the correction table built in the FMC for VNAV approaches? The ISA deviation is already there, at least on the 737.

[411A]

Would it be too much asking to have the correction table built in the FMC for VNAV approaches?
Can do...just supply the dollars.:hmm:

By the way, the explanation supplied by enginefire is concise and correct.

[aterpster]

ant1:

Would it be too much asking to have the correction table built in the FMC for VNAV approaches? The ISA deviation is already there, at least on the 737.

You will note that Jepp charts state, "...for uncompensated Baro-VNAV..."

Stateof-the-art, high-end FMSes now have compensated Baro-VNAV, thus the limitation does not apply for those systems. Nor does the limitation apply when flying an LNAV/VNAV IAP with a WAAS navigation system.

[FE Hoppy]

Even the cheap honeywell fms on the E-jets has temperature compensation .

[ant1]

Accordind to FCTM I understand that I would still have to make those corrections if I were further up North. :{

When appropriate, make cold temperature altitude corrections by applying a
correction from an approved table to the Waypoint altitude constraints. The FMC obtains the GP angle displayed on the LEGS page from the navigation database. This GP angle is based on the standard atmosphere and is used by the FMC to calculate the VNAV path which is flown using a barometric reference. When OAT is lower than standard, true altitudes are lower than indicated altitudes. Therefore, if cold temperature altitude corrections are not made, the effective GP angle is lower than the value displayed on the LEGS page. When cold temperature altitude corrections are made, VNAV PTH operation and procedure tuning function normally, however, the airplane follows the higher of the glide path angle associated with the approach (if available) or the geometric path defined by the waypoint altitude constraints.

Note: Temperature corrections redefine the glide path only if the FMC has the
geometric path option installed. Reference the applicable FCOM for
optional equipment installation.

[engfireleft]

That's interesting. I don't fly the 737 and can't put the section you quoted in context so I don't really know what kind of approach they are talking about here. But I do know that for a vertically guided RNAV approach the procedure must be selected from the database, and neither the waypoints nor procedure altitudes may be modified in any way (except for inserting VAPP at the FAF). That isn't an operating technique either, it's a certification requirement.

[ant1]

FCTM again:

For approaches where an RNP is specified, or approaches where a DA(H) is used, the waypoints in the navigation database from the FAP onward may not be modified except to add a cold temperature correction, when appropriate, to the waypoint altitude constraints.

[engfireleft]

Is the temperature correction generated by the FMC itself, or can you get it from a separate source like a temperature correction chart? The reason I ask is that the FAA AC's specify that it can only come from the FMC itself. Are you in Europe working under JAA rules?

[ant1]

I'm under "carbon copy" JAA rules and my understanding is that I would have to input manually the corrections from the tables (see 1st FCTM quote). Also from the first FCTM quote if I don't put temp corrections, the higher built in glide path angle will be flown.

Let's see what useful information this thread can produce.

[FE Hoppy]

ant1,
The GP angle will be lower if you don't use temp corrected heights. The reason being that the final approach leg will start at a lower geometric height due to the temperature error. The end height being much smaller is less effected by the temperature error.

[ant1]

Should be as you say if it weren't for what my understanding of the 1st quote is: ... however, the airplane follows the higher of the glide path angle associated with the approach (if available) or the geometric path defined by the waypoint altitude constraints.

[FE Hoppy]

When cold temperature altitude corrections are made, VNAV PTH operation and procedure tuning function normally, however, the airplane follows the higher of the glide path angle associated with the approach (if available) or the geometric path defined by the waypoint altitude constraints.

You missed the first part.

[reynoldsno1]

As an adjunct to efl's excellent post, the designer producing an LNAV/VNAV procedure takes into account the lowest recorded temperature at a particular aerodrome, rounds it down to the nearest 5deg and calculates the temperature error at the glide path intercept altitude. This lower altitude is then used to calculate an obstacle identification surface (OIS) - any obstacle that penetrates is used to calculate the DA by adding an appropriate clearance.

If the intercept altitude is more than 5NM from the threshold, then the equivalent ILS "W" surface is used instead of the calculated OIS - which is an even lower surface (about 1.63deg)

[Denti]

The technology replacing LNAV/VNAV on the 737 is IAN, using IAN a cold temperature correction is not permittet.


IAN Requirements and Restrictions

• airplanes must be equipped with FMC U10.5 or later and IAN FMA displays
• dual or single engine approaches are authorized
• raw data monitoring is required during localizer based approaches. During FMC based non-ILS approaches, raw data monitoring is recommended when available in accordance with the techniques described in the Non-ILS approach section in this chapter
• QFE operation is not authorized
• cold temperature altitude corrections are not permitted
• RNP appropriate for the approach must be used
•the autopilot is required until suitable visual reference is established when performing an approach requiring an RNP of 0.15 or lower.

[ant1]

FE Hoppy: yes I did. I had to read that several times. I wish I had IAN.