What do you do if you lose speed indication?

[downscc 1,327]

12 hours ago, PopsBellNC said:

Being a continuous, life-long, student (i.e., you never graduate) of the "What If? School of Flight", a 10 year-old Garmin eTrex handheld tossed in your bag can provide a surprising amount of useful information in this situation. These are the types of things that paranoid pilots play with (recall: "Paranoia is Perfect Awareness!" ) while 6 hours out over the Pacific. 😉

I have one of those stories.  In the C-414 ILS to Dallas Redbird IMC the HSI goes on vacation and I continue on that old handheld Garmin.  Couple of caveats first is that I was still cross checking instruments and flying partial panel so the Garmin was simply an additional tool, second the overcast was about 600 AGL so I was not going to push it to the limit,  I'd be visual with plenty of room if needed to clean up any mess.  That was an interesting approach.

[BlueStar 323]

3 hours ago, downscc said:

I continue on that old handheld Garmin. 

Dan,

Bigger cojones than I have. 🤣

Wasn't there an accident in Bristol, TN (maybe Martinsville) where the pilot was relying on a non-certified GPS and flew it into the mountain?

Grace and Peace, 

[downscc 1,327]

3 minutes ago, Bluestar said:

Dan,

Bigger cojones than I have. 🤣

Wasn't there an accident in Bristol, TN (maybe Martinsville) where the pilot was relying on a non-certified GPS and flew it into the mountain?

Grace and Peace, 

The LOC/GS deviation indicators on the HSI were still working, the failure was in the compass card, or directional gyro.  The GPS was only used to maintain a given track.  Not many mountains within 600 nm of Dallas.  The primary vertical obstacles are the tall antennas several miles SW of Redbird and I know the area well.

[PopsBellNC 21]

4 hours ago, downscc said:

That was an interesting approach.

Dang - I bet it was!! 😳

Edited November 9, 2018 by PopsBellNC
"bad" word... ;)

[BlueStar 323]

36 minutes ago, downscc said:

Not many mountains within 600 nm of Dallas

Or downtown Dallas. 🙂

Grace and Peace, 

[berts 97]

43 minutes ago, Bluestar said:

Dan,

Bigger cojones than I have. 🤣

Wasn't there an accident in Bristol, TN (maybe Martinsville) where the pilot was relying on a non-certified GPS and flew it into the mountain?

Grace and Peace, 

There have been several CFIT accidents in the past where the pilot has apparently relied too much on a single flight instrument or GPS which has resulted in the loss of the aircraft.  

One example of this was to a Beech 95-B55 Baron G-RICK. The aircraft was cleared for an ILS/DME approach to Runway 09 at Dundee Airport. Although it was raining and windy, the weather conditions were suitable for the approach. The pilot made a radio transmission reporting he was 4 nm to the west of the runway, on final approach, but the aircraft failed to arrive. A Search and Rescue helicopter located the aircraft wreckage on high ground 6.7 nm west of the airport. The investigation determined that it was likely that the instrument approach, carried out in IMC, was flown using range information based on the GPS distance from the ‘DND’ NDB, located 2.6 nm west of the airport, instead of using DME distance from the runway.

Although it was determined that this accident was not related to loss of airspeed information, neverthless it shows the importance of not relying on a single piece of information to determine the aircraft's current position and maintaining situational awareness at all times, especially when flying in Instrument conditions.

[paulyg123 135]

Is my analysis correct here

Well say you just took of and climbing through 4000 ft.  You are probably on VNAV and that is controlling your speed to 230-250 kts.   You are climbing at 2000 ft/min.  Flaps already retracted.  If you speed indication is fails here and your speed drops can this leading to a fatal stall. I guess at level flight you’ll notice a loss with speed as you plane pitches op to maintain altitude.  But if you are climbing and on autopilot,then the situation becomes harder to detect?

[downscc 1,327]

2 hours ago, paulyg123 said:

Is my analysis correct here

Well say you just took of and climbing through 4000 ft.  You are probably on VNAV and that is controlling your speed to 230-250 kts.   You are climbing at 2000 ft/min.  Flaps already retracted.  If you speed indication is fails here and your speed drops can this leading to a fatal stall. I guess at level flight you’ll notice a loss with speed as you plane pitches op to maintain altitude.  But if you are climbing and on autopilot,then the situation becomes harder to detect?

With all the redundancies you are proposing a very unlikely scenario.  Worst case you still have the backup instruments.

However, to answer what the hypothetical pilot response would be, as taught during basic flight training, is to use the attitude indicator, altimeter and heading gyro to fly the aircraft without an airspeed indication.  You speed would only drop if you increased the pitch or decreased the power so the pilot will use the pitch and altimeter with power setting appropriate for desired flight (climb, level or descend). I am providing a basic airmanship answer, the airline folks are going to have QRH and checklists and CRM all going on as well but the basic airmanship is the same.  Level off, assess, probably return to origin or takeoff alternate.

[skelsey 765]

On 11/10/2018 at 2:06 PM, paulyg123 said:

Is my analysis correct here

Well say you just took of and climbing through 4000 ft.  You are probably on VNAV and that is controlling your speed to 230-250 kts.   You are climbing at 2000 ft/min.  Flaps already retracted.  If you speed indication is fails here and your speed drops can this leading to a fatal stall. I guess at level flight you’ll notice a loss with speed as you plane pitches op to maintain altitude.  But if you are climbing and on autopilot,then the situation becomes harder to detect?

As Dan says, attitude (out the window/on the ADI) + power (EPR/N1) still = performance. This is why it is important to know what is "normal" for a particular aeroplane at any particular stage of flight.

For example, in the initial climb with flaps retracted you will have climb thrust set and perhaps 10-12 degrees nose up to give about 250kt. Above FL100 you will still have climb thrust set and a slightly lower nose attitude, perhaps 7 degrees nose up, to give a higher airspeed.

You are right that if the airspeed is unreliable then so will be the autopilot and depending on the failure mode this can be a gotcha. MSFS/P3D simulates a blocked pitot with a clear drain hole so the pitot pressure is equal to the static pressure and so is the IAS. If the autopilot were coupled in a speed on elevators mode here a pitch down would be commanded in an effort to regain IAS. However, there are other failure modes in real life - for instance, a totally blocked pitot would cause the ASI to behave like an altimeter - showing an increase in IAS as the aircraft climbs, and this would result in a pitch up to regain airspeed if the autopilot were left engaged in, say, FLCH or VNAV SPD.

In any case the answer is always to identify the erroneous readings and then use the reliable side if any, again by comparing the instrument readings with the pitch attitude and power setting. The L, C and R autopilots will normally use inputs from different pitot/static systems and the QRH will direct you to disengage the autopilot on the affected side and use only the ones using unaffected inputs, if possible.

I know some pilots like to note down the pitch and N1 on the OFP at each waypoint when doing their fuel checks so that they have a reference for what is right if something does happen.