Brachos 30a - Davening While Seated

The gemara discusses a situation where one has to take a trip early in the morning before the z'man krias shema. Their choice is to either stand while davening but forfeit being סומך גאולה לתפלה or to daven while travelling. The Shulchan Aruch (99:8) rules that it is better to daven before leaving one's home while standing, rather than davening on the road walking, even though it will be at the expense of being סומך גאולה לתפלה. M.B. cites Magen Avrohom who seems to hold that it is better to daven on the road when one can be סומך גאולה לתפלה.

The beginning of the sugya seems to refer to the virtue of "standing" to the exclusion of walking, not to the exclusion of sitting. This is actually very meduyak in Rashi. Rashi throughout the sugya comments that the advantage of davening while standing is for the purpose of intent. This makes a lot of sense a tthe beginning of the sugya when we are contrasting "standing" to walking.

ד"ה תפלה מעומד - שיכול לכוין את לבו לפיכך היו מקדימין להתפלל מעומד בביתם שלא יצטרכו להתפלל בדרך במהלך

However, as the sugya progresses we are told that R. Ashi would daven during his shiur from his seat, implying that the issue due to the fact that he is sitting rather than standing. Yet, Rashi continues his approach to explain the advantage of "standing" to be one of intent.

ד"ה בהדי צבורא - כמו שהיה יושב בתוך הצבור, כלומר, שאינו יוצא, וכי הוה אתי לביתיה חוזר ומתפלל מעומד לכוין את לבו

ד"ה לא חזינן - לפיכך מתפלל מיושב במקומו ולפי שאין דעתו מיושבת עליו כ"כ צריך לחזור ולהתפלל בביתו

It seems that Rashi understands that this sugya does not address the virtue of standing over sitting, rather it discusses the issue of davening in a situation where one won't be as focused. Both when one is travelling and when Rav Ashi was delivering his lecture, they were unable to properly focus on the tefillah. Rashi understands that the reason Rav Ashi davened again at home was not because the first time he sat through his davening (actually Rashi interprets רב אשי מצלי בהדי צבורא מיושב doesn't necessarily mean he was sitting, rather it means he davened in his place without walking out of the shiur where he could concentrate better). According to Rashi's approach in the sugya, we have no source for the advantage of davening while standing vs. sitting.

Tosafos in the name of Rabbeinu Meir learns from Rav Ashi that if one did not daven while standing, they must repeat the shemoneh esrei when they get home while standing. However, from Tosafos as well we don't see that the issue was the fact that one sat instead of standing, rather the issue is that if one davened in a situation where they couldn't concentrate, they are required to repeat when they can concentrate. Furthermore, see Rabbeinu Yona who explains the sugya also to be one of concentration and therefore writes that the issue is for one to be walking or riding, but sitting on an animal would be no worse than standing.

Yet, the Shulchan Aruch 94 speaks about the importance of davening while standing, not leaning or sitting. In si'if 9 he writes that if one is forced to sit for shemone esrei, they must repeat the shemoneh esrei later while standing. The Beis Yosef points to Tosafos as the source of this halacha. However, a careful reading of Tosafos doesn't compel any advantage to standing over sitting, and certainly doesn't require repeating shemone esrei. The Shulchan Aruch seems to be taking the Rambam's approach that the issue being discussed in our sugya is standing vs. sitting, and rules like Tosafos that if one didn't do what they were supposed to, they must daven again (but tosafos themselves will say that the issue is about concentration, not sitting).

The Mishna Berura (94:27) writes that the prevalent minhag is that if one had to daven while seated they don't daven again later, against the ruling of Shulchan Aruch. In light of what we explained that most Rishonim understand the issue in the sugya to be a lack of concentration rather than physical position, and in light of the Rambam who explicitly says that if one davened while sitting they would not need to repeat, the minhag is to rely on the tefillah one davened while seated. Therefore, if one davened on an airplane while seated, they don't need to daven again when they get off the plane.