What do you do if you lose speed indication?

[paulyg123 135]

I was flying and had erratic speed readings on my 777.  (Probably a weather issue with ASN or switching PMDG plane).  But in real life, if you lose your speed on a 777, is there a back up instrument that gives you speed from a redundant source?  Can you ask ATC for your speed?  I know they have speed on their radars, but it that a result of the plane's transponder data? or from the ATC radar blip.

If not, what do you do if you have no clue of your speed?

 

 

[Budbud 227]

Hi,

If you cannot rely or completely loose your speed indication, then you have to follow the QRH section that gives you the pitch and thrust to maintain for each step of the flight.

In the QRH, in the chapter Performance Inflight, you are provided with the data for each aircraft model.
Have a look page page 737&738 for the -300ER for instance.

I wouldn't ask the speed the ATC since they would give you a ground speed and not an air speed. Depending on the winds, it may result in being way off the flight enveloppe.

Edited November 4, 2018 by Budbud

[DavidP 1,408]

Power + Attitude = Performance.

Second flight school lesson

 

[BlueStar 323]

59 minutes ago, paulyg123 said:

I was flying and had erratic speed readings on my 777.

In the military I was taught "pitch and power".  I always know how fast the aircraft is going in level flight at a specific power setting.  Determining power varies from engine to engine and can be MP, N1, EPR, Torque.  Level flight can be determined with VSI, Altimeter, Attitude Indicator, Compass, or Airspeed. If I'm not climbing or descending and I'm not turning, My three primary instruments are Power (the most important), compass, and altimeter.  In my instrument scan I look at compass and altimeter every scan and the others every second or third scan. In your example if I were level and had erratic airspeed and the power was unchanged I would go to the standby airspeed indicator and my FO's instruments verified with the altimeter and power.

In my civilian career I have never had a problem with any of my flight instruments (other than one stick shaker).  My military career was a different story where I've had numerous flight instruments fail due to external circumstances. 🙂   

Grace and Peace, 

[Chock 17,769]

Prior to losing the speed indication (and from experience in general of flying the type) you're likely to be aware of the correct throttle and pitch settings for maintaining flight and as others have noted, this is a standard procedure technique for if you lose ASI. It's what the crew should have done on Air France Flight AF447 - an Airbus A330 which had lost its static port and pitot readings in icing conditions and had the AP disconnect as a result of the data loss and then started giving erroneous speed warnings - but unlike with a Boeing, on an Airbus there ain't no motorised throttle, only detents for various autothrottle settings, so getting used to the correct throttle position is easier on a Boeing than it is on an Airbus. But AF447 did nevertheless highlight this as an issue where training might have been lacking and too much reliance on automation led to a deterioration in these kind of 'seat of the pants' skills which pilots of old, and pilots who fly aeroplanes with less automation tend to have more of.

Although not completely analogous - since there is wind speed to take into account too - the standard technique to deal with this is nevertheless really not too different from what you might do if the speedometer broke in your car; i.e. in your car you'd have a rough idea of where your right foot normally is on the gas pedal to do various speeds, so if you held a familiar setting, you'd know your were 'about right' speed-wise. It wouldn't be spot on, but it'd be close enough. Another thing you can do in a car under those circumstances, and to some extent you can do in an aeroplane, is to use position data to gauge your speed, in the same way that a sat nav in a car can give you your speed from calculating the change in your GPS position, however, that is ground speed and not airspeed so it's not going to be perfect, but in an emergency, it's better than nothing since you would hopefully have at least some idea of the wind speed.

Having said that, if you had the time to do it and there were enough ground based nav aids available, it would be possible to calculate speed and also your wind drift from triangulating your changing position with your radios and the compass (or GPS data), which would give you the wind speed data necessary to be able to determine your airspeed reasonably accurately.

This is the sort of thing evil IFR instructors like to do to you - i.e. sticking post it notes over various instruments - and then seeing how you deal with it, so if you're ever doing IFR training, try and do it in a tandem-seat aircraft rather than one with side-by-side seating, that way they can't reach your panel lol!

Edited November 4, 2018 by Chock

[FS++ 707]

2 hours ago, paulyg123 said:

I was flying and had erratic speed readings on my 777.  (Probably a weather issue with ASN or switching PMDG plane).  But in real life, if you lose your speed on a 777, is there a back up instrument that gives you speed from a redundant source?  Can you ask ATC for your speed?  I know they have speed on their radars, but it that a result of the plane's transponder data? or from the ATC radar blip.

If not, what do you do if you have no clue of your speed?

 

 

In theory, you can work out the speed on modern airliners with INS or IRS by looking at FMC CDU They even teach that in flight school..

Now, after a whole career flying the thing without incident and adjusting to automation, I challenge you to remember that one bit of information at 2 AM in the morning, when you probably didn't even sleep the night before due to Jetlag from outbound flight.

.

 

[downscc 1,327]

Nobody has mentioned this so I'll toss it out:  Pilot and copilot flight instruments are redundant systems and the standby instrument(s) is/are a 3d independent source.  You basically figure out which one to rely on and fly with it.  Of course, all other discussion is relevant too.  Partial panel flying, where there is a little round "soap holder" stuck to an instrument, was how I spent most of my instrument training.  One learns to fly with any variation of instruments failed.  The big difference between the training and real world is detecting the problem and sorting it out. 

Edited November 4, 2018 by downscc

[paulyg123 135]

Downscc.  Are you saying on a recent Boeing plane the pilot and copilots speeds are determined by separate systems?  If that is the case I need not worry about losing my speed readings,  I’d just check with my co pilot.   Let me know if  this is the case

[downscc 1,327]

FCOM 10.20.9 and the systems description in that chapter.  You have four different pitots, three air data computers and a standby airspeed indicator that displays raw data.  Notice the rotary switch on the lower left, left of the PFD, each side can independently select alternate air data computers. 

If you suspect a false reading then you have two other air data computer channels (one of which your FO is probably using) and a standby instrument.  You also have the pitch and thrust to compare with those sources.

[Ephedrin 418]

It won‘t help you on a 777 but in a glider airplane you have only one airspeed indicator... if it fails you fly after your ear. 😉 

 

in a big airliner, if EVERYTHING fails, no redundancy anymore available this is an emergency of the bad part... then your experience is asked and quite a bit of luck... and this thing that can‘t be added to any QRH, manual or lesson: feel. Look out, feel your controls, hear the wind... and pray xD

Edited November 4, 2018 by Ephedrin

[berts 97]

33 minutes ago, paulyg123 said:

Are you saying on a recent Boeing plane the pilot and copilots speeds are determined by separate systems?  If that is the case I need not worry about losing my speed readings,  I’d just check with my co pilot.   Let me know if  this is the case 

It is safer if the pilots carry out any checks like this independently and then compare.

If you are in severe icing conditions or have no pitot heaters working then you could end up losing all of your airspeed readings and, as others have already said, this is where knowing what power and attitude to set to maintain level flight becomes very important. There are usually tables in the onboard manuals which the pilots can refer to in order to extract this sort of data, but as Dan has said, it is important to determine which instrument has failed and which ones, if any, are still reliable.  If all else fails - and it is obviously not your day to go flying - you should always respect any stick shaker or stall warning if they activate.

There was a B747 accident some years ago where the Captain's attitude indicator (HSI) failed and slowly toppled shortly after takeoff from Mumbai.  Apparently the fault was caused by a failed transistor, but instead of comparing their primary instruments with the standby artificial horizon to determine which two out of the three were accurate, the pilots followed the faulty one and unfortunately the aircraft subsequently crashed in the sea.       

[Chock 17,769]

2 hours ago, paulyg123 said:

Are you saying on a recent Boeing plane the pilot and copilots speeds are determined by separate systems?

This is pretty much the case for every airliner. Usually there is a selector switch in the cockpit (often located on the overhead on the left side) to select which source the instruments on either side of the panel are being fed from. On the outside of the aeroplane there are static ports and pitot tubes for the pilot's side and the co-pilot's side, plus a standby pitot tube and an alternate static port source too.

Below is a pic of where the pitots are on an Airbus A330, they are in a roughly similar location on the Boeing Triple Seven. Although not the case on this picture, there is quite often stenciling alongside the pitots to indicate which one feeds which instrument in order to assist with maintenance, although it is invariably the case that you can tell which one is which by which side of the aeroplane it is on:

Not visible on this picture, but also on airliners near the nose on both sides you will see an area with a large stenciled box around it (usually stencilled in either red or black depending on the aeroplane's livery) with a hole located in the middle of that; this is the static port. The stenciled box area around it indicates that the area has to be kept scrupulously clean.

How the system works, is the static port is a sideways-facing hole (so it is not affected by airflow), and this reads the current air pressure, it does so by having sensors attached to a sealed thin metal box which has a vacuum inside it, the more air pressure there is, the more the box is compressed by air pressure. Movement sensors read this compression and can therefore work out the current air pressure, so it is basically just a very sophisticated and sensitive barometer. This barometric value is compared to the pressure being read by the intake of oncoming air at the pitot tubes; the difference in pressure between this and the static pressure measured by the static port, is how the airspeed is calculated. Obviously up at altitude it can be very cold, so these things can be heated to prevent ice from blocking them up and if the aeroplane is going to be on the ground for any appreciable length of time, they are covered up by very conspicuous 'remove before flight' tags to prevent insects and small birds from nesting in them. Believe it or not, a small wasp which nested in a pitot tube was actually the cause of an airliner crash a few years ago.

What is not apparent on this picture of the pitot tubes, is that at the back of the tube just before where it bends upwards to attach to the aeroplane, is a small drainage hole so that any water which enters the tube can escape out of the back of it instead of blocking it up.

The other thing you might see sticking out of the aeroplane skin near the pitot tubes and air temp probes, are the angle of attack vanes. These are usually a flat metal blade mounted on a swivel so that they can rotate in the airflow to indicate the AoA the aeroplane is at to angle sensors which are attached to them. Since these could freeze up too and be inhibited from rotating, they also often have provision for them to be heated up.

As you can see from the picture, unlike other things which stick out from the aeroplane all over the place, such as antennas and vent doors etc, instrument probes are not generally painted, instead being bare metal, quite often either galvanised steel or made of something like stainless steel or titanium to prevent corrosion. Because they can be heated and can also get very cold too, painting them would rather pointless unless you went to the trouble of using specialist paint on them which could handle a very wide temperature range, so it's just easier to not bother painting the things at all.

Although these probes are often too high up on the fuselage to present a hazard to ground personnel, on smaller airliners such as a CRJ or an ATR, you have to be careful not to touch them because if they have their heating switched on, they are so hot that your skin would stick to them if it touched them. Having said that, you can actually just about touch these ones on the A330, and so you have to be careful not to do that when opening the panel to connect the GPU/FEP. We actually saw one these probes (a few nights ago on an Airbus A330 that had just taxied in at Manchester) which had a black smoke scar trailing back from it along the fuselage for a few inches, which gives you an idea of how hot they can get, although that evidence of smoke on the aircraft skin which we spotted was unusual enough for us to report it as a potential issue when we did the walk around check, as it doesn't normally look like that.

Edited November 5, 2018 by Chock

[BlueStar 323]

1 hour ago, berts said:

attitude indicator (HSI) failed and slowly toppled

That is why I cross-check with the RMI/Compass to verify. If I ain't turning then my wings are level.  

I realize my military training was different than civilian training, and this is not a criticism of civilian instruction.  One of the maneuvers that I was required to perform on partial panel was to do a spin and roll out on a designated heading.  It has been my experience that even the best attitude indicators will tumble on occasion.  

A little trivia for those who may not know, when performing a split S and rolling in on a target, the bottom of all military attitude indicators that I am familiar with reads "DIVE". 🤣

Grace and Peace, 

[paulyg123 135]

Excellent responses.  I really learned a lot here.  That is why I love this forum.

[BrianW 124]

3 hours ago, paulyg123 said:

Are you saying on a recent Boeing plane the pilot and copilots speeds are determined by separate systems?

Not exactly if we’re talking about a 777.  In the 777 both PFDs are receiving data from the ADIRU.  This data is voted data from the air data modules (3 each pitot and static).  Single channel air data is only displayed on its respective PFD if ALTN is selected as the instrument source.