MACH or IAS?
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MACH or IAS?
Hi there,
Let's say you requested for a 2000ft descent from FL370 to FL350 due turbulence, you want to do a controlled rate of descent and so you elected to use V/S. Usually with V/S, speed window will be opened, do you keep it at Mach or IAS?
My thought process is, going by the Mach = TAS/speed of sound formula, I'll elect to target IAS so as not to exceed MMO. What's your thoughts?
Thanks in advance.
Let's say you requested for a 2000ft descent from FL370 to FL350 due turbulence, you want to do a controlled rate of descent and so you elected to use V/S. Usually with V/S, speed window will be opened, do you keep it at Mach or IAS?
My thought process is, going by the Mach = TAS/speed of sound formula, I'll elect to target IAS so as not to exceed MMO. What's your thoughts?
Thanks in advance.
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Well assuming this is a 737? To be honest I'd usually use Alt Intervent as that should give a nice 1000fpm descent assuming this was done prior to TOD calculated by the VNAV.<br />If for whatever reason I needed VS over 2000 feet unless you were hugging the barbers pole mach is fine. Remember the barbers pole will move away from your current IAS as you start to descend.
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Fair enough but IAS will be creeping up. (Although i'm quite sure the chances of it reaching VMO by just descending 2000ft is rare)...
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At those flight levels you should stay in MACH and shouldn't worry about MMO overspeed unless:
- You were already cruising at a Mach close to MMO e.g M 0.8 if MMO was M 0.82 or :
-WInd aloft are quite strong and during descent you expect a vertical windshear that wil accelerate your aircraft aerodynamically and increase the Indicated Mach like a significant tailwind decrease ex : At FL370 tailwind 70 Kts and at FL350 tailwind 50 Kts that will certainly reasult in an overspeed .
- You were already cruising at a Mach close to MMO e.g M 0.8 if MMO was M 0.82 or :
-WInd aloft are quite strong and during descent you expect a vertical windshear that wil accelerate your aircraft aerodynamically and increase the Indicated Mach like a significant tailwind decrease ex : At FL370 tailwind 70 Kts and at FL350 tailwind 50 Kts that will certainly reasult in an overspeed .
The other thing that may be worth doing is discussing with a training Captain what actually is going to happen if you exceed MMO by 5 knots, 10 knots , 15 knots etc
Talk to an Engineer about what inspections they need to do when the aircraft is oversped by different amounts.
Read up on how the aircraft is certified and how fast the Boeing test pilots fly it when determining speeds.
After doing that I am pretty sure that you will come to the conclusion that overspeeding the aircraft by a few knots is something that you will obviously try to avoid, but at the same time is no big deal. The safety of the flight is not at risk if the aircraft momentarily exceeds MMO in the situation you have described.
Now.......go and look at the risks involved with getting slow while at altitude...different kettle of fish.
Talk to an Engineer about what inspections they need to do when the aircraft is oversped by different amounts.
Read up on how the aircraft is certified and how fast the Boeing test pilots fly it when determining speeds.
After doing that I am pretty sure that you will come to the conclusion that overspeeding the aircraft by a few knots is something that you will obviously try to avoid, but at the same time is no big deal. The safety of the flight is not at risk if the aircraft momentarily exceeds MMO in the situation you have described.
Now.......go and look at the risks involved with getting slow while at altitude...different kettle of fish.
It would be rare that you fly that close to MMo/Vmo for that to be of any concern.
Especially since your reason for descending is turbulence.
Your speed would already be rolled back and I would descend in FLC or simply scroll the altitude down and push the botton to recruise in VNAV.
Especially since your reason for descending is turbulence.
Your speed would already be rolled back and I would descend in FLC or simply scroll the altitude down and push the botton to recruise in VNAV.
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If you had "selected" Mach then presumably a Mach would be indicated, so the Mach would only increase if the selected V/S was too high for the aircraft to maintain the present Mach number, intervention would be required. The same would happen in IAS.
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DESCENDING FRom F370 to F350 due turbulence if I am not in an airspace committed separation by MACH I would revert to SPD personally primarily because I would at constant pitch and Spd mode descent my F/F will reduce in that 2000' ft band might as well take advantage of that. Secondly spd has more specific selection of spd down to the single knot. & thirdly MMO excedence probability would be lesser compared to Mach mode descent if a high v/s is chosen to descend. After level off WIL be back to MACH. However on a given day, the scenario will dictate the best selection 
Originally Posted by Extricate
Fair enough but IAS will be creeping up. (Although i'm quite sure the chances of it reaching VMO by just descending 2000ft is rare)...
I don't fly the 737, but as you are in Mach at 370 and you will end up cruising in Mach when level at 350, descend in Mach and VS -1000fpm, or B2N2s FMS mode.
Originally Posted by 73qanda
After doing that I am pretty sure that you will come to the conclusion that overspeeding the aircraft by a few knots is something that you will obviously try to avoid, but at the same time is no big deal.
Yes for sure a tech log write up and it will need to be signed off by an Engineer ( degree of inspection depends on magnitude of over speed) so I take your point about a maintenance base. I guess what I was trying to point out is that it is to be avoided, but don’t be scared of it, keep it in perspective. I find myself flying with more and more people who appear to be scared of a small overspeed and that drives behaviours ( closing thrust levers) that aren’t warranted. The same pilots appear happy to slow down to low Mach numbers at high flight levels. I was trying to encourage the OP to research what the actual consequences are of both scenarios. There is plenty of good info on the net by reputable organisations. There are many examples of slow flight leading to the crew having to descend but not many of incidents caused by small MMO exceedances.
There are lots of different ways of getting down, it probably doesn’t matter which method as long as the speed and ROD flown are sensible, I do however see two reasons not to implement B2N2 ‘s FLC advice.
There are lots of different ways of getting down, it probably doesn’t matter which method as long as the speed and ROD flown are sensible, I do however see two reasons not to implement B2N2 ‘s FLC advice.
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73qanda I'm 100% with you. Always amazed me how some pilots will happily fly close to Green Dot speed at very high altitude in turbulence... MMO is a aerodynamic limitation not a structural limitation. it's almost impossible to exceed the structural limitation in cruise phase with an overspeed. Talking about the A320, during certification they pushed the aircraft up to 381kts IAS (VD) and Mach .89 (MD) and it was perfectly fine. Source: Safety First #21 January 2016. The VMO/MMO is 350/.82 on A320 Family. Yes at low level we can overtake most airliners xD
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If this is B737NG specific, then remember your aircraft is happy flying in the low drag bucket of 40-260KIAS. If you were in ECON, at F370, depepnding on weight you'd have been at M0.77? Come down to F350 and the ECON would be about M0.755 or similar. Neither is anywhere near limiting and to be honest if you left it in MACH then the resulting speed at F350 is still less than LRC, the top end of that efficient low drag range of KIAS.
Me? I'd just ALT Intervene and let the VNAV deal with it. It will select the ECON cruise speed and adjust to it on the way down. The biggest problem with V/S is generally the lack of auto changeover. Put simply, in MACH, your KIAS goes up in descent, in IAS, the MACH decreases. But then you should know that.
Me? I'd just ALT Intervene and let the VNAV deal with it. It will select the ECON cruise speed and adjust to it on the way down. The biggest problem with V/S is generally the lack of auto changeover. Put simply, in MACH, your KIAS goes up in descent, in IAS, the MACH decreases. But then you should know that.
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furthermore, the FCTM contains enough guidance on VMO/MMO exceedance limitations. It is actually preferred to exceed a MMO/VMO limitation due to sudden wind speed changes at altitude instead of disconnecting the AP and manually correcting!!
off course, when you are descending from FL370 to FL 350 due turbulence, whether you do that in Mach or IAS(Mach of course would be normal) it would be wise to reduce that speed anyways!!
Last edited by flyburg; 29th Jul 2018 at 21:51.
Talking about the A320, during certification they pushed the aircraft up to 381kts IAS (VD) and Mach .89 (MD) and it was perfectly fine.
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Exceeding mach number does not result in exceeding structural load limits and does not damage the aircraft. However, exceeding critical Mach number may result in a nose tuck and THAT will result in all sorts of bad structural mojo. Slightly exceeding MMO for long periods of time may result in early fatigue failure of the wing, tail surface, or other important structure. This is because the structure may experience large numbers of cyclic loads caused by buffet which will fatigue the structure. The extent of fatigue will depend on the duration and magnitude of the buffet.
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Why would you overspeed? The barberpole will move up at the same rate as the increase in IAS when flying a MN. To end up with a selected MN into VMO takes a while. The only thing that could lead to overspeed is a change in atmospheric conditions. As you start your descent the a/c will increase the rate to maintain the increasing MN so you can end up with a high rate for only a 2000 ft descent and that could potentially cause an overspeed or a TCAS. Thats why I prefer VS instead of open des/lvl chg for small altitude changes.
Repeating what many have said already, in a slightly different way. An aicraft that flies at high altitudes and high Machs will have a "Mach Transition" or "crossover" altitude. Varies a bit with the day's weather (and aircraft type), but it is the altitude where Vmo (in IAS) equals Mmo (in Mach number). For most aircraft today that is in the region of FL260, although I have seen a few mentions, for particular airframes, of FL300. (and for Concorde, ~51300 feet, I believe - 440 knots all the way up to 51300, and Mach 2.02 higher than that - and coming down, 350 knots until 34-35000 feet, where the aircraft dropped below Mach 1).
Above that altitude, you fly with Mach as the speed reference, and by definition, if you do not exceed Mmo, you will never exceed Vmo.
Below that altitude, you fly with IAS as the speed reference/limit, and by definition, if you do not exceed Vmo, you will never exceed Mmo.
My impression, not having tried them all, is that today's FMCs handle the transition automatically - the aircraft "knows" its own limits, and will use whichever speed is lower (strictest limit). With an optional manual IAS/M selection button.
Above that altitude, you fly with Mach as the speed reference, and by definition, if you do not exceed Mmo, you will never exceed Vmo.
Below that altitude, you fly with IAS as the speed reference/limit, and by definition, if you do not exceed Vmo, you will never exceed Mmo.
My impression, not having tried them all, is that today's FMCs handle the transition automatically - the aircraft "knows" its own limits, and will use whichever speed is lower (strictest limit). With an optional manual IAS/M selection button.
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Always amazed me how some pilots will happily fly close to Green Dot speed at very high altitude in turbulence..
