flyingsquirrel

Monitor the descent with mental arithmetic to confirm whether the VNAV programming is sensible and whether the wind/energy assumptions remain valid. Take the altimeter reading in hundreds of feet and multiply by 3 (e.g. FL370 = 37 * 3 = 111). Take the middle digit of your IAS (e.g. 270 kts = 7) and add it on. Take the tailwind at altitude (from the map display) and divide it by 10. Add that on. If engine anti ice will be used during the descent add a ballpark figure of 5 miles per 10,000'. So for FL300, 50 kt tailwind, descending at 270kts and engine anti-ice from FL300 to FL50, the required track miles is 90 + 5 + 7 +12.5 = 114.5 nm. Correctly program the VNAV with the target speeds (including the desired speed over the beacon or on a radar-vectored base), assuming 180kts for flap 5 will steer you in the right direction. The descent winds and engine anti-ice have a bearing and must be entered, otherwise it's garbage in - garbage out and you've wasted 5 minutes of your life. Some real world targets that work (the Boeing FCTM is a little optimistic in the descent planning section, particularly for the -800): - 10,000' 250kts, 40nm track miles - 12nm out, up speed, 3000' AGL. Continually monitor track mileage and height using height x 3: check head or tailwind on final for configuration planning. - At glideslope intercept: flap 5 and '5' speed For a low drag approach, assuming a 3 degree glideslope, select gear down at 2300', flap 15, complete the landing checklist, at 1300', select landing flap. Things to catch you out: - straight in approaches with long finals (>12nm) as you'll have to slow down and configure flap whilst descending. Intercepting the glideslope clean or at flap 5 but high on speed will mean that prompt action is required - depending on how far you've got left to descend. The speedbrake on the -800 is ineffective at low speed but sometimes the 1-300fpm difference that it makes will be enough. Look at the descent page on the FMC to see whether your flight path angle is greater than the Vertical Bearing - if it is then you'll be wresting control back from the gods. The real option for slow down when in the dwang is the gear and the best philosophy for avoiding an unstable approach is to grab the aeroplane by the neck and make it do what you want. VNAV is a wonderful trap. It only shows you height above or below the optimum descent path: the path that you have programmed. If there are hard heights in there then they may be above an ideal glide descent profile - so temporarily delete hard heights to see where you actually want to be (useful heads-up for many STARs). What VNAV pays no attention to though is energy, i.e. you can be on path but flying 330kts - when the path was programmed for 270kts or 240 below FL100. Try and imagine the VNAV deviation as a colour, if you're fast then it's red and if you're slow then it's blue. Red needs drag or increased speed to dive below the path. Consider the path deviation as miles, using 300' per mile, so if you're 9000' high on profile, you'll need 27 extra miles or have to dive down to get below and then create sufficient track miles to slow back down onto your planned speed. 2100' below the path gives you 7 miles. Last thing on slowing down: in level flight it's about .75kt per second. In the descent it's about .375kt/second. So, quick sum: 250 kts to up speed ~ 45 kts. 60 seconds in level flight, 120 seconds in descent. 250 kts is about 4 miles per minute. 250-up in level flight = 4 miles. 250-up in descent = 8 miles. So the amount to be below path if aiming to slow up to up speed and regain the path again is approximately 2400'. Use these same numbers for higher up to enable more control and also appreciation of what VNAV is actually telling you. Lastly, VNAV can also bite when you turn inside the lateral path that you've programme for an arrival. Also, if the lateral arrival has a turn of more than 60 degrees then the aircraft will turn inside it. Radar vectoring may also turn you inside the lateral path. The result: a reduction in track miles. If you were 40 miles out then it's not an issue but if you're 12 miles out and "suddenly" you're 9 miles out then you're 1000' high and likely looking for localiser and glideslope intercept. The world has changed to a bright shade of red and it's fighting the energy once again - just when you think that you're home and dry. The tip for this: update the lateral path and anticipate what reduction in track miles there's going to be and be at the correct speed and correct height. These days, most approaches, particularly precision approaches, are flown as continuous descent, and this means either meticulous programming of VNAV to create a shallow descent path to decelerate and configure, and/or, flying in V/S mode and adjusting the descent rate depending on the track miles.