And these files are surprisingly small. William Cecil, marketing manager of data acquisition and wireless solutions at Teledyne Controls, estimates that the flight recorder on an Airbus A320 or Boeing 737NG accumulates 1.8 MB of unformatted data per flight hour.
Packets of raw engine data amount to as much for an entire flight. Even so, potentially useful flight data is frequently minimized, postponed or forgotten.
Why are operators so miserly with MRO data? Because ACARS is narrow-band ("an extremely slow, skinny pipe," Teledyne's Schmitz called it) and pricey. Messages are limited to blocks of 220 characters, one block at a time, up to a maximum of 16.
Throughput in VHF mode is 2.4 kilobits per second (kbps), the rate of an antique modem. Even VDL Mode-2, a recently introduced VHF variant with 10 times the bandwidth, clocks in at only 30 to 40 kbps.
Usage charges vary with a variety of considerations, including message size and communications channel, and prices have been falling under pressure from VDL-2 and Iridium's new satcom option. Still, the range is roughly five cents a message (VHF) to between $1.50 and $20 per minute (satcom).
In addition, Bob Bouchard, vice president of technology solutions at The Longbow Group, questioned ACARS's reliability: "A Wi-Fi or cellular connection uses standard TCP/IP. If a file is skipped, it resends it.
If it's missing, you'll know. With ACARS, you send a message hoping there's a ground receiver and really no acknowledgement it was received."
Given these technical limits, the possibility of using ACARS for sizable or continuous MRO data transfers is, William Cecil said, "a dream."
...
In-Flight Datalinks
Data rate per second, Range
ACARS (Satcom)9.6kglobal
ACARS (VHF)2.4k240 mi
ACARS (HF)0.18k5,000 mi
ACARS VDL Mode-2 (VHF)31.5k240 mi
from:
OverHaul & Maintenance, Jan 1, 2008