HIGH ALTITUDE TAKE-OFF & LANDINGS

[Guest november678x]

Hello to all, I am a bit lost on this topic. I know the FMC that PIC has (I assume its Americans model) calculates V1, Vr, V2, and Vref speeds automatically. Does this FMC calculate speed based only on weight? Which would give inaccurate speeds if you land or take-off at a high altitude airport like Denver, because the speeds will vary with altitude and temperature (correct me if I am wrong). Or does the FMC know the pressure altitude of the airport that you put in the route page? Also I can understand for take-off if the FMC takes the current outside temperature for Vspeed calculation, but how can the FMC take into account the temperature of the destination airport for speed calculation? I don

[Guest]

Hello,I've often wondered that. I just checked, and in the file "wpNavAPT.txt" in the FMCWPnavdata directory there is one entry for each runway in the database, including this one for KDEN 17R. Notice the last four digits in the enry are "5431." That is the altitude in feet for Denver Int'l Airport (a little above the rest of the Mile High City)Denver Intl KDEN17R12000169 39.861244-104.658489108.5016905431 Each individual runway for each individual airport in the FMC database has a specific line in the txt file and they all end in a four digit number, coresponding with the FS altitude of the airport. In real life the runways begin and end at different altitudees obviously, but as of yet this is not modelled in FS2002, and I wouldn't suspect that it would be very high on the priority list for FS2004.Peace in the New Year from Denver.Later,Neil

[Guest november678x]

Hi thanks for the response, but what about the real FMC on the 767? Thank You. :-)

[Guest Martin]

Landning speeds are not dependant on altitude or temperature, as far as I understand.I'm not sure whether the PIC FMC actually takes altitude and temp into account when calculating takeoff speeds.Martin767 fetishistIt's a lot like life and that's what's appealing

[Guest ilh]

My understanding is that V speeds are independent of altitude. The ram air pressure that the airspeed indicator uses to measure indicated airspeed is the same air pressure felt by the wings.Yes, longer takeoff and landing runs are needed at higher altitudes, but this is because true airspeed is higher at higher altitudes. For a given indicated Vr, at sea level the true Vr will be much lower than that at 10,000'.Lee Hetherington (KBED)

[marcom]

Vr and V2 are independent of altitude.You are mixing two things up. Groundspeed and Airspeed. At sea level, with no winds and standard temperature, GS = IAS. The higher you get, the higher the GS will become at the same airspeed. The airspeed indicator directly shows you how many air molecules are jammed (rammed) into the pitot tube. It's the same molecules that support your wing. Thus, flying at 250 kts IAS at sea level will be the same (performance wise) as flying at 250 kts IAS at FL180.V1 on the other hand could very well be altitude dependent, as it takes much longer to accelerate at higher altitudes than at lower altitudes to the same airspeed (due to the higher groundspeed necessary and less air entering the turbines). So V1 could be pretty low at high altitudes if your runway is not longer.Regards,Mark

[Guest november678x]

Ok please correct me if I am wrong on anything here. The FMC on the real 767 for this FMC model that American has (I assume) calculates VRef speeds, Vr, V2 speeds based on weight only, temperature and altitude are not taken into account in any way. So if the V1 is based on altitude and temperature how does the FMC know the altitude the current airport to automatically calculate V1? Also I see a field that says Ref Temp on the take-off Ref page, is this where I you put in current temperature to get a V1 calculation? No matter what I put in there it does not change V1. Thank You:-)

[Guest Martin]

As far as I understand (from reading the AOM) the real FMC (the version that automatically calculates V speeds) takes gross weight, flap setting, thrust setting, runway position (i.e. taking off from an intersection instead of the end of the runway), runway slope, takeoff wind, temperature, and of course altitude into account (the runway altitude is known from the FMC database obviously) when calculating V speeds.And according to the AOM, all the takeoff speeds are dependant on these factors (not only V1). However, assuming otherwise identical conditions, the speeds will be the same up to about 4,500 ft altitude and 30

[Guest oyvindhansen]

The temperature entry is used for thrust calculation. If you set it to a lower temp than the outside temp the T/O thrust reference will be lower than maximum. V1 depends on at least AC weight and runway length, though I'm not sure if that is implemented in PIC.- Oyvind

[Guest november678x]

I believe that's the Reduce thrust entry, I meant on page 2 of the take-off ref page where the temperature is. It has about 1 degree less temp than outside I believe. What is that temperature field for? I can enter a temperature myself. Must you enter current temp for a takeoff calculation? Thank You :-)

[Guest november678x]

>runway position >(i.e. taking off from an intersection instead of the end of >the runway), runway slope,..Is that what the RWY/POS field is for? You enter Rwy then the position at which you start take-off run? Also in the rwy slope field, is this the percentage amount that is depicted on charts? For example 0.2% up. Thank You :-)

[Guest Martin]

>Is that what the RWY/POS field is for? You enter Rwy then >the position at which you start take-off run?Yes, you enter the distance from the runway threshold in hundreds of meters. Valid entries according to the manual are -30 to 30 (i.e. -3000 m to 3000 m).>Also in the >rwy slope field, is this the percentage amount that is >depicted on charts? For example 0.2% up. Thank You :-)The manual doesn't state whether this is percentage, only that valid entries are U.0 to U2.0 or D.1 to D2.0 (U means upslope and D downslope of course).Neither of these functions are modelled in PIC though.Martin767 fetishistIt's a lot like life and that's what's appealing

[Guest Martin]

Since the takeoff speeds are higher if the OAT is very high, it seems reasonable that OAT is taken into account when calculating V speeds.Martin767 fetishistIt's a lot like life and that's what's appealing

[marcom]

You can enter a so-called assumed temperature into this field. If you are departing from a very long runway (let's say 4000m) and you are nowhere near MTOW, with good weather, you won't need the same engine performance as when you are much higher. As performance decreases with temperature, you can enter an assumed temperature and the FMC will check an integrated database to match the performance for this temperature. You can enter a value up to 55 degrees Celsius for a 4000m runway and a short hop. Note, to use the new engine performance data, you must select N1 on the A/T panel prior to takeoff. This will set thrust to the calculated derated N1 value.Hope this helped,Mark

[Guest november678x]

So the Ref OAT field on page 2 of the take-off page is for the derated take-off thrust entry? I thought that went in the thrust field on page 1. Thank You:-wave