Post by Tom Goodrick on Jan 2, 2010 19:52:59 GMT -5
Here is an example of how the Cab Alt gauge works during a flight in the Learjet 35. The flight was made this afternoon using RW. The start was at Milwaukee's MKE airport and the destination was Huntsville, AL, KHSV (my home field). The sky was mainly clear for the flight but there were strong winds aloft, 80 to 100 knots across the path from the west. At times upsets caused by wind speed and direction changes caused disruption in the cabin altitude as I had to adjust power. The direction changes put the airspeed over the limit so I had to reduce power. Whenever you reduce power it reduces the bleed air pressure flow and that increases the cabin altitude with my gauge.
The aircraft is by Stone; the FD is mine. This model has a Cab_Alt_51 gauge.
min feet feet Real Weather for 1/2/10 2pm CST
FLT TM ALT CAB ALT REMARK
-10 723 723 engines off
-9.8 723 506
-9.5 723 206
-7.5 723 296
-5 723 254 adding power decreases cab alt
-1 723 270
0 723 105 takeoff power
3 11000 1972 brief wait at 11000
4.31 14640 1951
5.34 20000 2570
6.31 25000 3512
7.83 30000 4473
9.86 35000 4218 power increase
11.17 37500 5955
12.49 40000 5697
13.66 41000 6853
14.16 41000 8922 power adjustments for overspeed
18 41000 8608 248 KIAS, 495 KTAS 0.79MACH
21.01 41000 8583 HEADING 180 WIND 86 KTS FROM 302
28.46 41000 8570
30.36 41000 9088 WIND CHANGES IN SPEED AND DIRECTION
38.52 41000 9628 power adjustments, 64% to 60% throttle
48.13 41000 9428 power kept low because exceeding limit
50 41000 11193 to 50% before beginning descent
51.69 38000 10511 peak after each reduction
52.31 36700 11311 to 40%
55.05 31100 9588
55.22 30700 10370 to 30% over Nashville
57.06 27000 9080 steady descent at -2000 fpm
57.22 26600 9765 to 20%
59.99 21000 7457
61.45 18000 6293
62.45 16900 5925 very disruptive wind shear, 30 deg bank
64 14200 4887 330 KIAS delayed spoiler deploy
65.38 11000 3754 spoiler was deployed, now 23 nm out
66.81 7500 2542
67.6 5500 1858
68.44 3500 1175 11 nm out
68.89 3500 1163 A/P off, spoiler retract, gear down, on glide slope
72.1 633 247 Landed 131.3 KIAS, -81 FPM
72.6 633 247
73.1 633 236
74.6 633 245
75.1 633 247 before shut down
75.35 633 552
75.85 633 633
I have never looked this closely at the gauge performance. I thought it would be smoother. Obviously, a good controller would damp out some of these variations, particularly those associated with the power reductions during descent. You can grab this data and throw it into an Excel sheet and graph it. It looks a little messy compared to the idealized graph in the Pendleton book. I have no idea how much variation occurs in real aircraft. I know there is some variation.
The aircraft is by Stone; the FD is mine. This model has a Cab_Alt_51 gauge.
min feet feet Real Weather for 1/2/10 2pm CST
FLT TM ALT CAB ALT REMARK
-10 723 723 engines off
-9.8 723 506
-9.5 723 206
-7.5 723 296
-5 723 254 adding power decreases cab alt
-1 723 270
0 723 105 takeoff power
3 11000 1972 brief wait at 11000
4.31 14640 1951
5.34 20000 2570
6.31 25000 3512
7.83 30000 4473
9.86 35000 4218 power increase
11.17 37500 5955
12.49 40000 5697
13.66 41000 6853
14.16 41000 8922 power adjustments for overspeed
18 41000 8608 248 KIAS, 495 KTAS 0.79MACH
21.01 41000 8583 HEADING 180 WIND 86 KTS FROM 302
28.46 41000 8570
30.36 41000 9088 WIND CHANGES IN SPEED AND DIRECTION
38.52 41000 9628 power adjustments, 64% to 60% throttle
48.13 41000 9428 power kept low because exceeding limit
50 41000 11193 to 50% before beginning descent
51.69 38000 10511 peak after each reduction
52.31 36700 11311 to 40%
55.05 31100 9588
55.22 30700 10370 to 30% over Nashville
57.06 27000 9080 steady descent at -2000 fpm
57.22 26600 9765 to 20%
59.99 21000 7457
61.45 18000 6293
62.45 16900 5925 very disruptive wind shear, 30 deg bank
64 14200 4887 330 KIAS delayed spoiler deploy
65.38 11000 3754 spoiler was deployed, now 23 nm out
66.81 7500 2542
67.6 5500 1858
68.44 3500 1175 11 nm out
68.89 3500 1163 A/P off, spoiler retract, gear down, on glide slope
72.1 633 247 Landed 131.3 KIAS, -81 FPM
72.6 633 247
73.1 633 236
74.6 633 245
75.1 633 247 before shut down
75.35 633 552
75.85 633 633
I have never looked this closely at the gauge performance. I thought it would be smoother. Obviously, a good controller would damp out some of these variations, particularly those associated with the power reductions during descent. You can grab this data and throw it into an Excel sheet and graph it. It looks a little messy compared to the idealized graph in the Pendleton book. I have no idea how much variation occurs in real aircraft. I know there is some variation.