Altimeter or airspeed if you had to choose one in emergency

[Geoff]

I am planning to install the two screen Aspen system with redundant MFD.  In the third row I will have the required stand alone attitude indicator and I will have the 6th hole of the standard 6 pack available for one last backup.

 

I am leaning towards altimeter because in a worst case I could estimate airspeed by visual reference if VMC.

 

The question is should it be altimeter or airspeed, talk amongst yourselves...

[NotarPilot]

Airspeed, because if you're in said VMC, wouldn't you just as easily be able to estimate altitude? Airspeed seems more critical being that you need to worry more about a stall. If the houses get too big, then you're getting low. Just MHO.

[N201MKTurbo]

I have landed a few times without an airspeed because of bugs in the pitot. It is a little unnerving. I have never flown without an altimeter. It depends on weather you are IFR or VFR, day or night. If I was IFR I would opt for the altimeter. VFR I could get by without either, but I would probably opt for the airspeed.

[larryb]

Airspeed. I have other backup altitude on my iPad and 796

[Marauder]

I'm guessing this is a trick question. Neither. Because whatever took out both of your Aspens with battery backups (you are buying the extended battery in the MFD, correct?) had to related to the pitot static system failing and would most likely impact the mechanical ASI or altimeter. If it were purely an electrical issue, with the extended battery (which is required to do away with both airspeed or altimeter) you would have at least an hour to find a place to land.

 

[rob]

Airspeed definitely. Portables can give you GPS altitude but they can't do airspeed (only ground speed).

[John Pleisse]

I have the Aspen 2000 and I am doing a panel clean up this summer. I will likely add a certified replacement engine monitoring system as well. Facing the same dilemma (and I do not know JAA requirements), the real need is exactly what to leave IN the panel to maintain vfr capability. It would be easy to go all glass....in fact perfect.....until you are at that rural airport, late on a Sunday night, no services and you get a red x on any one of the screens. Now you are stuck, not legal VFR and need a ferry permit, even to make a short flight for repairs.

 

My answer, leave in your aircraft, whatever is needed to fly vfr, day and night, should you get a red x on any of the new systems. The redundancy would be a plus.

 

FAR 91.205.

 

In a pickle? I'll take the altimeter all day.

[Marauder]

Alt..... you can "hear" your airspeed. You can't hear a mountain in front of you.

 

I agree and even beyond, I for one know the power settings required to get the airspeed I want.

[Bob - S50]

Neither.  I'd save the weight.

 

If someone were holding a gun to my head and making me choose I'd pick airspeed.

 

Like someone said, I can get GPS altitude off my tablet and it runs on batteries.

 

As for airspeed, unless you plan on installing a second pitot tube, you would still have a single point failure that could take out both airspeed indicators.  The only thing you would have protection from would be the indicator itself.  How often do they fail?  Worst case, I fly GPS groundspeed and GPS altitude.  If I'm landing with a headwind, my TAS will be higher than my groundspeed so the airplane is not going to fall out of the sky.  I might float a bit but at that point who cares?  I'll only fall a foot anyway (an inch for the pros out there).

 

If you want something different, think about an AOA gauge instead.  That's independent of the pitot static system.   Or a Dynon D1 that gives you attitude, slip, GPS altitude, GPS rate of climb, and ground speed.  I'm assuming of course that you already have an engine monitor.

 

Bob

[PMcClure]

Altimeter. With an AI and Altimeter you should know your Airspeed within a few knots. There are back-up altimeter readings in the plane but having that dial winding down or winding up tells you a lot if you are caught in IMC. 

[rob]

Doesn't the trend on an airspeed indicator essentially tell you the same amount of information?

[AndyFromCB]

Leave airspeed in the hole. Here is why: TruTrak gemini.

[Geoff]

Bob,

The AOA is a good thought. I think I can fit an AOA elsewhere in the panel and then put the altimeter in the last hole thereby having effectively both!

Thanks.

[PTK]

If I had to pick one of the two I'd pick altimeter and here's why.

If we exclude consideration of VSI in descents, the altimeter is primary for pitch in nearly all flight situations. The only exception I can think of is constant AS climb. Not the entry where AI is primary but a stabilized climb at constant AS.

You can fly and land without an ASI.

[Cruiser]

First off the likelyhood of a dual Aspen system failure is about zero.

IF I have to pick, I would keep the altimeter. It can be certified and will help to keep you from hitting something.

there are lots of ways to estimate airspeed including preset MAP x RPM settings and a stall horn.

If I had to fly an instrument approach without the Aspens I would want to know for certain how high I was above the things that could hurt me..

[John Pleisse]

First off the likelyhood of a dual Aspen system failure is about zero.

 

So going full glass, now that the law allows it should have us turn our back on already installed analogue instrumentation? Seriously, I want to know. I feel the second I rip all of this old stuff out, I will have a problem with an Aspen or JPI. You know it is gonna happen. If it does...grounded. Rely on 1 electric bus and 2 batteries? Cruiser or anyone chime in. I am seriously weighing options myself and gutting my panel this summer.

[wishboneash]

Knowing the power settings and if the altimeter is not moving you know you are at constant airspeed. Reduce power and you are going to descend at the same airspeed. Increase power and you climb at the same airspeed. If the VSI is working that is a bonus. So altimeter is probably most critical. With the AI working you can adjust airspeed as well with a certain pitch up or pitch down attitude. an AoA indicator would be even better.

[Mooneymite]

Altimeter.

[Awful_Charlie]

There's some good thoughts above, and if it was me (and an option while still complying with the STC) I think I'd be tempted to junk the extra horizon, and have the ASI and Altimeter. If that's not an option, then I would try hard to make the third hole.

 

Justification - even if you have no electrical system, you can fly day VFR non-radio subject to minimum instruments, which my understanding is ASI and altimeter only. Without one of those, an electrical failure grounds you on landing until it's repaired

[Oscar Avalle]

There's some good thoughts above, and if it was me (and an option while still complying with the STC) I think I'd be tempted to junk the extra horizon, and have the ASI and Altimeter. If that's not an option, then I would try hard to make the third hole.

 

Justification - even if you have no electrical system, you can fly day VFR non-radio subject to minimum instruments, which my understanding is ASI and altimeter only. Without one of those, an electrical failure grounds you on landing until it's repaired

 

I agree with you. However, if no other option I would get let the altimeter go.

[Cruiser]

John, I don't see the risk or probability of the Aspen 2000 system failing completely. That is BOTH the primary PFD and secondary MFD units (which are totally independent) failing at the same time. With the REV function you are better protected than the Garmin systems since they do not have a reversion function built in. If a Garmin fails your SOL therefore you MUST retain the analog gauges.

 

I suppose you can argue that it MIGHT happen but then I would reply that you MIGHT loose all your analog instruments also, about the same risk of happening, probably higher.

 

Ben, As for the electrical failure grounding the airplane, I would say that is true regardless of instrumentation. Who here would knowingly take of for a pleasure flight with the electrical system inop? My plane requires an operating electrical system for VFR DAY operations anyway. I am pretty sure all Mooneys with electric gear do.

[Awful_Charlie]

I hear what you're saying Tony, but maybe it's just because I fly to a few islands and other more remote airfields where A&P's don't necessarily congregate ;-)

 

If the airspace and POH allows, and I can get the engine started, I'd consider flying NORDO to an A&P with the gear locked down, but no, I wouldn't do a pleasure or pax carrying flight for the hell of it without the electric system.  It's not an option in a Bravo, as the engine indicating instruments are electric, and they are required, but if you have non-powered gauges then why build in an extra dependency?

[201er]

Bob,

The AOA is a good thought. I think I can fit an AOA elsewhere in the panel and then put the altimeter in the last hole thereby having effectively both!

Thanks.

You guys beat me to it.

[jetdriven]

John, I don't see the risk or probability of the Aspen 2000 system failing completely. That is BOTH the primary PFD and secondary MFD units (which are totally independent) failing at the same time. With the REV function you are better protected than the Garmin systems since they do not have a reversion function built in. If a Garmin fails your SOL therefore you MUST retain the analog gauges.

I suppose you can argue that it MIGHT happen but then I would reply that you MIGHT loose all your analog instruments also, about the same risk of happening, probably higher.

Ben, As for the electrical failure grounding the airplane, I would say that is true regardless of instrumentation. Who here would knowingly take of for a pleasure flight with the electrical system inop? My plane requires an operating electrical system for VFR DAY operations anyway. I am pretty sure all Mooneys with electric gear do.

In my limited Aspen time I did have one fail for a very simple reason. It rained the night before and some water migrated into the static ports which blocked the static line at the drain. It was pretty surprising to see the screen go black and red-X. I imagine if you had two aspens connected to the same static source that could happen.

[jetdriven]

We flew it for about ten minutes before the attitude presentation came back. It remained on but the airspeed read low and the airspeed and altitude was unreliable for the rest of the 2 hour flight. The standby altimeter and ASI were similarly unreliable. I like the idea of dual AHRS with two aspens but I was rather shocked that a static line blockage takes down the entire Aspen. We only drained out about 5 drops of water from the rear static drain.

Our J has a curious, repeatable problem. After washing the plane the airspeed reads zero until flying 3-5 minutes with the pitot heat on. The airspeed also reads low in light rain after a few minutes. After flying 15 minutes in light rain it read 50 MPH low in cruise. Pitot heat brings it back to normal as well.